diff --git a/adminfront/src/features/users/UserCreatePage.tsx b/adminfront/src/features/users/UserCreatePage.tsx
index a8e6dfbf..d47a49c5 100644
--- a/adminfront/src/features/users/UserCreatePage.tsx
+++ b/adminfront/src/features/users/UserCreatePage.tsx
@@ -8,6 +8,8 @@ import {
   Plus,
   Save,
   Trash2,
+  Mail,
+  X,
 } from "lucide-react";
 import * as React from "react";
 import { useForm } from "react-hook-form";
@@ -58,7 +60,11 @@ import {
 import type { UserSchemaField } from "./userSchemaFields";
 import { resolvePersonalTenant } from "./utils/personalTenant";
 
-type UserFormValues = UserCreateRequest & { metadata: Record<string, unknown> };
+type UserFormValues = UserCreateRequest & {
+  metadata: Record<string, unknown> & {
+    sub_email?: string[];
+  };
+};
 type UserCategory = "hanmac" | "external" | "personal";
 
 type PickerTarget = { kind: "appointment"; index: number };
@@ -135,6 +141,8 @@ function UserCreatePage() {
   );
   const [isResolvingTenant, setIsResolvingTenant] = React.useState(false);
 
+  const [newSubEmail, setNewSubEmail] = React.useState("");
+
   const { data: tenantsData } = useQuery({
     queryKey: ["tenants", "all"],
     queryFn: () => fetchAllTenants(),
@@ -164,10 +172,35 @@ function UserCreatePage() {
       position: "",
       jobTitle: "",
       role: "user",
-      metadata: {},
+      metadata: {
+        sub_email: [],
+      },
     },
   });
 
+  const currentSubEmails = (watch("metadata.sub_email") as string[]) || [];
+
+  const handleAddSubEmail = (e: React.KeyboardEvent<HTMLInputElement>) => {
+    if (e.key === "Enter" || e.key === "," || e.key === " ") {
+      e.preventDefault();
+      const value = newSubEmail.trim().replace(/,/g, "");
+      if (value && value.includes("@") && !currentSubEmails.includes(value)) {
+        setValue("metadata.sub_email", [...currentSubEmails, value], {
+          shouldDirty: true,
+        });
+        setNewSubEmail("");
+      }
+    }
+  };
+
+  const handleRemoveSubEmail = (emailToRemove: string) => {
+    setValue(
+      "metadata.sub_email",
+      currentSubEmails.filter((e) => e !== emailToRemove),
+      { shouldDirty: true },
+    );
+  };
+
   // Lock company for tenant_admin
   React.useEffect(() => {
     if (profile?.role === "tenant_admin" && profile.tenantSlug) {
@@ -370,10 +403,15 @@ function UserCreatePage() {
     const {
       hanmacFamily: _hanmacFamily,
       userType: _userType,
+      sub_email: rawSubEmail,
       ...formMetadata
     } = data.metadata ?? {};
+
+    const sub_email = Array.isArray(rawSubEmail) ? rawSubEmail : [];
+
     const metadata: Record<string, unknown> = {
       ...formMetadata,
+      ...(sub_email.length > 0 ? { sub_email } : { sub_email: [] }),
     };
 
     const payload: UserCreateRequest = {
@@ -584,6 +622,73 @@ function UserCreatePage() {
               )}
             </div>
 
+            <div className="space-y-2">
+              <Label
+                htmlFor="sub_email_input"
+                className="flex items-center gap-2"
+              >
+                {t("ui.admin.users.create.form.sub_email", "보조 이메일")}
+              </Label>
+              <div className="flex flex-col gap-2">
+                <div className="flex flex-wrap gap-2 mb-1">
+                  {currentSubEmails.map((email) => (
+                    <div
+                      key={email}
+                      className="inline-flex items-center gap-1 rounded-md border px-2.5 py-0.5 text-xs font-semibold transition-colors focus:outline-none focus:ring-2 focus:ring-ring focus:ring-offset-2 bg-secondary text-secondary-foreground"
+                    >
+                      {email}
+                      <button
+                        type="button"
+                        onClick={() => handleRemoveSubEmail(email)}
+                        className="text-muted-foreground hover:text-foreground ml-1 rounded-full p-0.5 hover:bg-muted transition-colors"
+                      >
+                        <X size={12} />
+                      </button>
+                    </div>
+                  ))}
+                </div>
+                <div className="relative">
+                  <Input
+                    id="sub_email_input"
+                    value={newSubEmail}
+                    onChange={(e) => setNewSubEmail(e.target.value)}
+                    onKeyDown={handleAddSubEmail}
+                    className="pr-20"
+                    placeholder={t(
+                      "ui.admin.users.create.form.sub_email_placeholder",
+                      "추가할 이메일 입력 후 Enter",
+                    )}
+                  />
+                  <Button
+                    type="button"
+                    variant="ghost"
+                    size="sm"
+                    className="absolute right-1 top-1 h-8 text-xs font-bold"
+                    onClick={() => {
+                      const value = newSubEmail.trim().replace(/,/g, "");
+                      if (
+                        value &&
+                        value.includes("@") &&
+                        !currentSubEmails.includes(value)
+                      ) {
+                        setValue(
+                          "metadata.sub_email",
+                          [...currentSubEmails, value],
+                          { shouldDirty: true },
+                        );
+                        setNewSubEmail("");
+                      }
+                    }}
+                  >
+                    {t("ui.common.add", "추가")}
+                  </Button>
+                </div>
+                <p className="text-[10px] text-muted-foreground mt-1">
+                  * 여러 개 입력 가능. 입력 후 엔터를 눌러 추가하세요.
+                </p>
+              </div>
+            </div>
+
             <div className="space-y-2">
               <div className="flex items-center justify-between">
                 <Label htmlFor="password">
diff --git a/adminfront/src/features/users/UserDetailPage.tsx b/adminfront/src/features/users/UserDetailPage.tsx
index 897384d7..02f1a721 100644
--- a/adminfront/src/features/users/UserDetailPage.tsx
+++ b/adminfront/src/features/users/UserDetailPage.tsx
@@ -17,6 +17,7 @@ import {
   Shield,
   Trash2,
   Users,
+  X,
 } from "lucide-react";
 import * as React from "react";
 import {
@@ -93,7 +94,10 @@ import {
 import { resolvePersonalTenant } from "./utils/personalTenant";
 
 type UserFormValues = Omit<UserUpdateRequest, "metadata"> & {
-  metadata: Record<string, Record<string, string | number | boolean>>;
+  email: string;
+  metadata: Record<string, Record<string, string | number | boolean>> & {
+    sub_email?: string[];
+  };
 };
 type UserCategory = "hanmac" | "external" | "personal";
 
@@ -408,6 +412,30 @@ function UserDetailPage() {
   const isSelf = Boolean(profile?.id && user?.id && profile.id === user.id);
   const watchedStatus = watch("status");
 
+  const [newSubEmail, setNewSubEmail] = React.useState("");
+  const currentSubEmails = (watch("metadata.sub_email") as string[]) || [];
+
+  const handleAddSubEmail = (e: React.KeyboardEvent<HTMLInputElement>) => {
+    if (e.key === "Enter" || e.key === "," || e.key === " ") {
+      e.preventDefault();
+      const value = newSubEmail.trim().replace(/,/g, "");
+      if (value && value.includes("@") && !currentSubEmails.includes(value)) {
+        setValue("metadata.sub_email", [...currentSubEmails, value], {
+          shouldDirty: true,
+        });
+        setNewSubEmail("");
+      }
+    }
+  };
+
+  const handleRemoveSubEmail = (emailToRemove: string) => {
+    setValue(
+      "metadata.sub_email",
+      currentSubEmails.filter((e) => e !== emailToRemove),
+      { shouldDirty: true },
+    );
+  };
+
   const resetMutation = useMutation({
     mutationFn: (newPass: string) => updateUser(userId, { password: newPass }),
     onSuccess: (_, newPass) => {
@@ -614,6 +642,7 @@ function UserDetailPage() {
           : null);
 
       reset({
+        email: user.email || "",
         name: user.name,
         phone: user.phone || "",
         role: user.role,
@@ -628,11 +657,20 @@ function UserDetailPage() {
         grade: user.grade || "",
         position: user.position || "",
         jobTitle: user.jobTitle || "",
-        metadata:
-          (user.metadata as unknown as Record<
+        metadata: {
+          ...((user.metadata as unknown as Record<
             string,
             Record<string, string | number | boolean>
-          >) || {},
+          >) || {}),
+          sub_email: Array.isArray(user.metadata?.sub_email)
+            ? user.metadata.sub_email
+            : typeof user.metadata?.sub_email === "string"
+              ? user.metadata.sub_email
+                  .split(/[;,\n\r\t]/)
+                  .map((e) => e.trim())
+                  .filter((e) => e.includes("@"))
+              : [],
+        },
       });
       const isUserHanmacFamily = isHanmacFamilyUser(
         user,
@@ -751,16 +789,32 @@ function UserDetailPage() {
     const {
       hanmacFamily: _hanmacFamily,
       userType: _userType,
+      sub_email: rawSubEmail,
       ...safeMetadata
     } = cleanMetadata;
+    
+    // Parse sub_email
+    let sub_email: string[] = [];
+    if (typeof rawSubEmail === "string" && rawSubEmail.trim() !== "") {
+      sub_email = rawSubEmail
+        .split(/[;,\n\r\t]/)
+        .map((e) => e.trim())
+        .filter((e) => e.includes("@"));
+    }
+
     const metadata: Record<string, unknown> = {
       ...safeMetadata,
+      ...(sub_email.length > 0 ? { sub_email } : { sub_email: [] }),
     };
 
     const payload: UserUpdateRequest = {
       ...data,
       metadata,
     };
+    // email cannot be updated directly via this API in current backend implementation,
+    // so we delete it from payload if it spread
+    // @ts-ignore
+    delete payload.email;
     payload.role = undefined;
 
     if (userCategory === "personal") {
@@ -935,6 +989,16 @@ function UserDetailPage() {
                 <Mail size={14} className="text-primary/70" />
                 {user.email}
               </div>
+              {user.metadata?.sub_email &&
+                Array.isArray(user.metadata.sub_email) &&
+                user.metadata.sub_email.length > 0 && (
+                  <div className="flex items-center gap-1.5 bg-background px-2.5 py-1 rounded-full border">
+                    <Mail size={14} className="text-primary/40" />
+                    <span className="text-[10px] font-bold">
+                      +{user.metadata.sub_email.length}
+                    </span>
+                  </div>
+                )}
               {user.phone && (
                 <div className="flex items-center gap-1.5 bg-background px-2.5 py-1 rounded-full border">
                   <Shield size={14} className="text-primary/70" />
@@ -1055,6 +1119,80 @@ function UserDetailPage() {
                   </div>
                 </div>
 
+                <div className="grid gap-8 md:grid-cols-2 pt-6 border-t border-dashed">
+                  <div className="space-y-2 col-span-full">
+                    <Label
+                      htmlFor="sub_email_input"
+                      className="text-xs font-bold uppercase text-muted-foreground flex items-center gap-2"
+                    >
+                      <Mail size={14} />
+                      {t("ui.admin.users.detail.form.sub_email", "보조 이메일")}
+                    </Label>
+                    <div className="flex flex-col gap-2">
+                      <div className="flex flex-wrap gap-2 mb-1">
+                        {currentSubEmails.map((email) => (
+                          <Badge
+                            key={email}
+                            variant="secondary"
+                            className="flex items-center gap-1 px-2.5 py-1 text-xs font-medium"
+                          >
+                            {email}
+                            <button
+                              type="button"
+                              onClick={() => handleRemoveSubEmail(email)}
+                              className="text-muted-foreground hover:text-foreground ml-1 rounded-full p-0.5 hover:bg-muted transition-colors"
+                            >
+                              <X size={12} />
+                            </button>
+                          </Badge>
+                        ))}
+                      </div>
+                      <div className="relative">
+                        <Input
+                          id="sub_email_input"
+                          value={newSubEmail}
+                          onChange={(e) => setNewSubEmail(e.target.value)}
+                          onKeyDown={handleAddSubEmail}
+                          className="h-11 shadow-sm pr-20"
+                          placeholder={t(
+                            "ui.admin.users.detail.form.sub_email_placeholder",
+                            "추가할 이메일 입력 후 Enter",
+                          )}
+                        />
+                        <Button
+                          type="button"
+                          variant="ghost"
+                          size="sm"
+                          className="absolute right-1 top-1 h-9 text-xs font-bold"
+                          onClick={() => {
+                            const value = newSubEmail.trim().replace(/,/g, "");
+                            if (
+                              value &&
+                              value.includes("@") &&
+                              !currentSubEmails.includes(value)
+                            ) {
+                              setValue(
+                                "metadata.sub_email",
+                                [...currentSubEmails, value],
+                                { shouldDirty: true },
+                              );
+                              setNewSubEmail("");
+                            }
+                          }}
+                        >
+                          {t("ui.common.add", "추가")}
+                        </Button>
+                      </div>
+                      <p className="text-[10px] text-muted-foreground mt-1">
+                        {t(
+                          "msg.admin.users.detail.sub_email_help",
+                          "* 보조 이메일로도 로그인이 가능하며 계정 찾기 등에 활용될 수 있습니다.",
+                        )}
+                      </p>
+                    </div>
+                  </div>
+                </div>
+
                 <div className="grid gap-8 md:grid-cols-2 pt-6 border-t border-dashed">
                   <div className="space-y-2">
                     <Label
diff --git a/adminfront/src/features/users/components/UserBulkUploadModal.tsx b/adminfront/src/features/users/components/UserBulkUploadModal.tsx
index 7e9f81ec..1c473094 100644
--- a/adminfront/src/features/users/components/UserBulkUploadModal.tsx
+++ b/adminfront/src/features/users/components/UserBulkUploadModal.tsx
@@ -127,9 +127,9 @@ function hanmacEmailStatusClass(preview?: HanmacImportEmailPreview) {
 
 export const downloadUserTemplate = () => {
   const headers =
-    "email,name,phone,role,tenant_slug,department,grade,position,jobTitle,employee_id,tenant_slug1,department1,grade1,position1,jobTitle1,employee_id1";
+    "email,sub_email,name,phone,role,tenant_slug,department,grade,position,jobTitle,employee_id,tenant_slug1,department1,grade1,position1,jobTitle1,employee_id1";
   const example =
-    "user1@example.com,홍길동,010-1234-5678,user,tenant-slug,개발팀,수석,팀장,프론트엔드,EMP001,second-tenant,센터,책임,,Architecture,EMP002";
+    "user1@example.com,sub1@test.com;sub2@test.com,홍길동,010-1234-5678,user,tenant-slug,개발팀,수석,팀장,프론트엔드,EMP001,second-tenant,센터,책임,,Architecture,EMP002";
   const blob = new Blob([`${headers}\n${example}`], {
     type: "text/csv;charset=utf-8;",
   });
@@ -297,9 +297,9 @@ export function UserBulkUploadModal({
 
   const downloadTemplate = () => {
     const headers =
-      "email,name,phone,role,tenant_slug,department,grade,position,jobTitle,employee_id,tenant_slug1,department1,grade1,position1,jobTitle1,employee_id1";
+      "email,sub_email,name,phone,role,tenant_slug,department,grade,position,jobTitle,employee_id,tenant_slug1,department1,grade1,position1,jobTitle1,employee_id1";
     const example =
-      "user1@example.com,홍길동,010-1234-5678,user,tenant-slug,개발팀,수석,팀장,프론트엔드,EMP001,second-tenant,센터,책임,,Architecture,EMP002";
+      "user1@example.com,sub1@test.com;sub2@test.com,홍길동,010-1234-5678,user,tenant-slug,개발팀,수석,팀장,프론트엔드,EMP001,second-tenant,센터,책임,,Architecture,EMP002";
     const blob = new Blob([`${headers}\n${example}`], {
       type: "text/csv;charset=utf-8;",
     });
diff --git a/adminfront/src/features/users/utils/csvParser.ts b/adminfront/src/features/users/utils/csvParser.ts
index ec0cb2f5..db811b38 100644
--- a/adminfront/src/features/users/utils/csvParser.ts
+++ b/adminfront/src/features/users/utils/csvParser.ts
@@ -354,9 +354,8 @@ function applySecondaryEmailMetadata(
   value: string,
 ) {
   const emails = splitEmailTokens(value);
-  item.metadata.sub_email = value;
-  item.metadata.secondary_emails = uniqueEmails([
-    ...metadataEmailList(item.metadata.secondary_emails),
+  item.metadata.sub_email = uniqueEmails([
+    ...metadataEmailList(item.metadata.sub_email),
     ...emails,
   ]);
   addWorksmobileAliasEmails(item, emails);
diff --git a/adminfront/tests/users_bulk.spec.ts b/adminfront/tests/users_bulk.spec.ts
index dd0bdd25..f87d02be 100644
--- a/adminfront/tests/users_bulk.spec.ts
+++ b/adminfront/tests/users_bulk.spec.ts
@@ -302,10 +302,7 @@ test.describe("Users Bulk Upload", () => {
     const payload = JSON.parse(bulkPayload);
     expect(payload.users[0].tenantSlug).toBe("primary-tenant");
     expect(payload.users[0].metadata.employee_id).toBe("EMP001");
-    expect(payload.users[0].metadata.sub_email).toBe(
-      "dual.alias@hanmaceng.co.kr",
-    );
-    expect(payload.users[0].metadata.secondary_emails).toEqual([
+    expect(payload.users[0].metadata.sub_email).toEqual([
       "dual.alias@hanmaceng.co.kr",
     ]);
     expect(payload.users[0].metadata.aliasEmails).toEqual([
diff --git a/adminfront/tests/users_bulk_secondary.spec.ts b/adminfront/tests/users_bulk_secondary.spec.ts
new file mode 100644
index 00000000..d7a03e7b
--- /dev/null
+++ b/adminfront/tests/users_bulk_secondary.spec.ts
@@ -0,0 +1,95 @@
+import { expect, test } from "@playwright/test";
+
+test.describe("Users Bulk Upload Secondary Emails", () => {
+  test.beforeEach(async ({ page }) => {
+    await page.addInitScript(() => {
+      window.localStorage.setItem("locale", "ko");
+      window.localStorage.setItem("admin_session", "fake-token");
+      (window as Window & typeof globalThis & { _IS_TEST_MODE?: boolean })._IS_TEST_MODE = true;
+
+      const authData = {
+        access_token: "fake-token",
+        token_type: "Bearer",
+        profile: { sub: "admin-user", name: "Admin", role: "super_admin" },
+        expires_at: Math.floor(Date.now() / 1000) + 36000,
+      };
+      window.localStorage.setItem("oidc.user:http://localhost:5000/oidc:adminfront", JSON.stringify(authData));
+    });
+
+    await page.route("**/api/v1/user/me", async (route) => {
+      return route.fulfill({
+        json: { id: "admin-user", name: "Admin", role: "super_admin", manageableTenants: [] },
+        headers: { "Access-Control-Allow-Origin": "*" },
+      });
+    });
+
+    await page.route("**/api/v1/admin/tenants*", async (route) => {
+      return route.fulfill({
+        json: { items: [], total: 0, limit: 100, offset: 0 },
+        headers: { "Access-Control-Allow-Origin": "*" },
+      });
+    });
+
+    await page.route("**/api/v1/admin/users*", async (route) => {
+      if(route.request().url().includes("/bulk")) {
+        return route.continue();
+      }
+      return route.fulfill({
+        json: { items: [], total: 0, limit: 50, offset: 0 },
+        headers: { "Access-Control-Allow-Origin": "*" },
+      });
+    });
+
+    await page.route("**/oidc/**", async (route) => {
+      await route.fulfill({ json: { issuer: "http://localhost:5000/oidc" } });
+    });
+  });
+
+  test("should parse secondary_emails and send to backend", async ({ page }) => {
+    let bulkPayload: any = null;
+
+    await page.route("**/api/v1/admin/users/bulk", async (route) => {
+      if (route.request().method() === "POST") {
+        bulkPayload = route.request().postDataJSON();
+        return route.fulfill({
+          json: { results: [{ email: "test@example.com", success: true, userId: "u-1" }] },
+          headers: { "Access-Control-Allow-Origin": "*" },
+        });
+      }
+      return route.continue();
+    });
+
+    await page.goto("/users");
+    await expect(page.getByTestId("page-title")).toContainText(/사용자|Users/i, { timeout: 20000 });
+
+    await page.getByTestId("user-data-mgmt-btn").click();
+    await page.getByRole("menuitem", { name: /일괄 임포트|일괄 등록|Bulk Import/i }).click();
+
+    // Create a mock CSV with secondary_emails
+    const csvContent = `email,sub_email,name,phone,role,tenant_slug\ntest@example.com,sub1@test.com;sub2@test.com,홍길동,010-1234-5678,user,tenant-slug`;
+
+    const fileChooserPromise = page.waitForEvent('filechooser');
+    await page.getByText(/파일 선택|Change file|Select file/i).click();
+    const fileChooser = await fileChooserPromise;
+
+    await fileChooser.setFiles({
+      name: 'users_with_secondary.csv',
+      mimeType: 'text/csv',
+      buffer: Buffer.from(csvContent),
+    });
+
+    await expect(page.getByText(/파싱 중/)).not.toBeVisible();
+    await expect(page.getByTestId("bulk-start-btn")).toBeEnabled();
+
+    await page.getByTestId("bulk-start-btn").click();
+
+    await expect(page.getByText(/성공|Success/i)).toBeVisible();
+
+    expect(bulkPayload).not.toBeNull();
+    expect(bulkPayload.users).toHaveLength(1);
+    
+    // The most important check - does it parse to the metadata
+    expect(bulkPayload.users[0].metadata.sub_email).toContain("sub1@test.com");
+    expect(bulkPayload.users[0].metadata.sub_email).toContain("sub2@test.com");
+  });
+});
\ No newline at end of file
diff --git a/backend/internal/domain/user.go b/backend/internal/domain/user.go
index 9195a3a5..83a7eb63 100644
--- a/backend/internal/domain/user.go
+++ b/backend/internal/domain/user.go
@@ -156,7 +156,7 @@ func (u *User) BeforeCreate(tx *gorm.DB) (err error) {
 }
 
 // ValidateLoginID checks if the loginID violates any collision, length, or security rules.
-func ValidateLoginID(loginID, email, phone string) error {
+func ValidateLoginID(loginID string, emails []string, phone string) error {
 	loginID = strings.TrimSpace(loginID)
 	if loginID == "" {
 		return nil
@@ -170,8 +170,10 @@ func ValidateLoginID(loginID, email, phone string) error {
 		return fmt.Errorf("ID cannot be an email format")
 	}
 
-	if email != "" && strings.EqualFold(loginID, email) {
-		return fmt.Errorf("ID cannot be the same as the email address")
+	for _, email := range emails {
+		if email != "" && strings.EqualFold(loginID, email) {
+			return fmt.Errorf("ID cannot be the same as the email address")
+		}
 	}
 
 	if phone != "" {
diff --git a/backend/internal/domain/user_validate_test.go b/backend/internal/domain/user_validate_test.go
index 9bf977d0..02117912 100644
--- a/backend/internal/domain/user_validate_test.go
+++ b/backend/internal/domain/user_validate_test.go
@@ -8,30 +8,31 @@ func TestValidateLoginID(t *testing.T) {
 	tests := []struct {
 		name    string
 		loginID string
-		email   string
+		emails  []string
 		phone   string
 		wantErr bool
 	}{
-		{"Empty", "", "test@email.com", "01012345678", false},
-		{"Valid alphanumeric", "user123", "test@email.com", "01012345678", false},
-		{"Too short", "us", "test@email.com", "01012345678", true},
-		{"Too long", "thisisaverylongloginidthatiswayoverthirtycharacters", "test@email.com", "01012345678", true},
-		{"Email format", "user@domain.com", "test@email.com", "01012345678", true},
-		{"Exact email match", "Test@Email.Com", "test@email.com", "01012345678", true},
-		{"Phone number match", "010-1234-5678", "test@email.com", "01012345678", true},
-		{"Phone number match +82", "+821012345678", "test@email.com", "01012345678", true},
-		{"Phone number match digits", "01012345678", "test@email.com", "01012345678", true},
-		{"Phone format (11 digits)", "01098765432", "test@email.com", "01012345678", true},
-		{"Valid pure digits (employee ID)", "20230001", "test@email.com", "01012345678", false},
-		{"Valid pure digits long", "123456789", "test@email.com", "01012345678", false},
-		{"Valid pure digits 10 chars", "1234567890", "test@email.com", "01012345678", false},
-		{"Reserved word admin", "ADMIN", "test@email.com", "01012345678", true},
-		{"Reserved word root", "root", "test@email.com", "01012345678", true},
+		{"Empty", "", []string{"test@email.com"}, "01012345678", false},
+		{"Valid alphanumeric", "user123", []string{"test@email.com"}, "01012345678", false},
+		{"Too short", "us", []string{"test@email.com"}, "01012345678", true},
+		{"Too long", "thisisaverylongloginidthatiswayoverthirtycharacters", []string{"test@email.com"}, "01012345678", true},
+		{"Email format", "user@domain.com", []string{"test@email.com"}, "01012345678", true},
+		{"Exact email match", "Test@Email.Com", []string{"test@email.com"}, "01012345678", true},
+		{"Secondary email match", "sub@test.com", []string{"test@email.com", "sub@test.com"}, "01012345678", true},
+		{"Phone number match", "010-1234-5678", []string{"test@email.com"}, "01012345678", true},
+		{"Phone number match +82", "+821012345678", []string{"test@email.com"}, "01012345678", true},
+		{"Phone number match digits", "01012345678", []string{"test@email.com"}, "01012345678", true},
+		{"Phone format (11 digits)", "01098765432", []string{"test@email.com"}, "01012345678", true},
+		{"Valid pure digits (employee ID)", "20230001", []string{"test@email.com"}, "01012345678", false},
+		{"Valid pure digits long", "123456789", []string{"test@email.com"}, "01012345678", false},
+		{"Valid pure digits 10 chars", "1234567890", []string{"test@email.com"}, "01012345678", false},
+		{"Reserved word admin", "ADMIN", []string{"test@email.com"}, "01012345678", true},
+		{"Reserved word root", "root", []string{"test@email.com"}, "01012345678", true},
 	}
 
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
-			err := ValidateLoginID(tt.loginID, tt.email, tt.phone)
+			err := ValidateLoginID(tt.loginID, tt.emails, tt.phone)
 			if (err != nil) != tt.wantErr {
 				t.Errorf("ValidateLoginID() error = %v, wantErr %v", err, tt.wantErr)
 			}
diff --git a/backend/internal/handler/auth_handler.go b/backend/internal/handler/auth_handler.go
index 6ae4c895..85b98c1d 100644
--- a/backend/internal/handler/auth_handler.go
+++ b/backend/internal/handler/auth_handler.go
@@ -374,7 +374,7 @@ func (h *AuthHandler) CheckLoginID(c *fiber.Ctx) error {
 	}
 
 	// Basic validation via our ValidateLoginID helper (without email/phone since we just check format & collision with reserved words)
-	if err := domain.ValidateLoginID(req.LoginID, "", ""); err != nil {
+	if err := domain.ValidateLoginID(req.LoginID, []string{}, ""); err != nil {
 		return c.JSON(fiber.Map{"available": false, "message": err.Error()})
 	}
 
@@ -801,7 +801,7 @@ func (h *AuthHandler) Signup(c *fiber.Ctx) error {
 	// Validate all collected LoginIDs
 	if collectedIDs, ok := attributes["custom_login_ids"].([]string); ok {
 		for _, lid := range collectedIDs {
-			if err := domain.ValidateLoginID(lid, req.Email, normalizedPhone); err != nil {
+			if err := domain.ValidateLoginID(lid, []string{req.Email}, normalizedPhone); err != nil {
 				return errorJSON(c, fiber.StatusBadRequest, "Invalid LoginID ("+lid+"): "+err.Error())
 			}
 		}
@@ -1769,7 +1769,7 @@ func collectEmailList(traits map[string]any, primaryEmail string) []string {
 		}
 	}
 
-	if raw, ok := traits["secondary_emails"]; ok {
+	if raw, ok := traits["sub_email"]; ok {
 		switch value := raw.(type) {
 		case []string:
 			for _, email := range value {
@@ -7953,7 +7953,7 @@ func (h *AuthHandler) UpdateMe(c *fiber.Ctx) error {
 	userPhone := extractTraitString(traits, "phone_number")
 	if collectedIDs, ok := traits["custom_login_ids"].([]string); ok {
 		for _, lid := range collectedIDs {
-			if err := domain.ValidateLoginID(lid, userEmail, userPhone); err != nil {
+			if err := domain.ValidateLoginID(lid, []string{userEmail}, userPhone); err != nil {
 				return errorJSON(c, fiber.StatusBadRequest, "Invalid LoginID ("+lid+"): "+err.Error())
 			}
 		}
diff --git a/backend/internal/handler/user_handler.go b/backend/internal/handler/user_handler.go
index b2cdf848..03faf338 100644
--- a/backend/internal/handler/user_handler.go
+++ b/backend/internal/handler/user_handler.go
@@ -757,7 +757,7 @@ func (h *UserHandler) CreateUser(c *fiber.Ctx) error {
 	// Validate all collected LoginIDs
 	if collectedIDs, ok := attributes["custom_login_ids"].([]string); ok {
 		for _, lid := range collectedIDs {
-			if err := domain.ValidateLoginID(lid, email, normalizePhoneNumber(req.Phone)); err != nil {
+			if err := domain.ValidateLoginID(lid, []string{email}, normalizePhoneNumber(req.Phone)); err != nil {
 				return errorJSON(c, fiber.StatusBadRequest, "Invalid LoginID ("+lid+"): "+err.Error())
 			}
 		}
@@ -1224,8 +1224,22 @@ func (h *UserHandler) BulkCreateUsers(c *fiber.Ctx) error {
 		// Validate all collected LoginIDs
 		if collectedIDs, ok := attributes["custom_login_ids"].([]string); ok {
 			valid := true
+			// Collect all emails
+			allEmails := []string{userEmail}
+			if secondaryRaw, exists := item.Metadata["sub_email"]; exists {
+				if secondaryEmails, ok := secondaryRaw.([]interface{}); ok {
+					for _, se := range secondaryEmails {
+						if seStr, ok := se.(string); ok {
+							allEmails = append(allEmails, seStr)
+						}
+					}
+				} else if secondaryEmails, ok := secondaryRaw.([]string); ok {
+					allEmails = append(allEmails, secondaryEmails...)
+				}
+			}
+
 			for _, lid := range collectedIDs {
-				if err := domain.ValidateLoginID(lid, userEmail, userPhone); err != nil {
+				if err := domain.ValidateLoginID(lid, allEmails, userPhone); err != nil {
 					results = append(results, bulkUserResult{Email: userEmail, OriginalEmail: emailEvaluation.OriginalEmail, SuggestedEmail: emailEvaluation.SuggestedEmail, Status: emailEvaluation.Status, Warnings: emailEvaluation.Warnings, Success: false, Message: "Invalid LoginID (" + lid + "): " + err.Error()})
 					valid = false
 					break
@@ -2036,9 +2050,23 @@ func (h *UserHandler) UpdateUser(c *fiber.Ctx) error {
 	// Validate all collected LoginIDs
 	userEmail := extractTraitString(traits, "email")
 	userPhone := extractTraitString(traits, "phone_number")
+	
+	allEmails := []string{userEmail}
+	if secondaryRaw, exists := traits["sub_email"]; exists {
+		if secondaryEmails, ok := secondaryRaw.([]interface{}); ok {
+			for _, se := range secondaryEmails {
+				if seStr, ok := se.(string); ok {
+					allEmails = append(allEmails, seStr)
+				}
+			}
+		} else if secondaryEmails, ok := secondaryRaw.([]string); ok {
+			allEmails = append(allEmails, secondaryEmails...)
+		}
+	}
+
 	if collectedIDs, ok := traits["custom_login_ids"].([]string); ok {
 		for _, lid := range collectedIDs {
-			if err := domain.ValidateLoginID(lid, userEmail, userPhone); err != nil {
+			if err := domain.ValidateLoginID(lid, allEmails, userPhone); err != nil {
 				return errorJSON(c, fiber.StatusBadRequest, "Invalid LoginID ("+lid+"): "+err.Error())
 			}
 		}