Improve user validation failed message

Changelog: changed
MR: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/80304
Issue: https://gitlab.com/gitlab-org/modelops/anti-abuse/anti-abuse-updates/-/issues/14

app/models/user.rb

@@ -2166,7 +2166,7 @@ class User < ApplicationRecord
   end
 
   def signup_email_invalid_message
-    self.new_record? ? _('is not allowed for sign-up.') : _('is not allowed.')
+    self.new_record? ? _('is not allowed for sign-up. Please use your regular email address.') : _('is not allowed. Please use your regular email address.')
   end
 
   def check_username_format

locale/gitlab.pot

@@ -43463,7 +43463,7 @@ msgstr ""
 msgid "is not a valid X509 certificate."
 msgstr ""
 
-msgid "is not allowed for sign-up."
+msgid "is not allowed for sign-up. Please use your regular email address."
 msgstr ""
 
 msgid "is not allowed for this group."
@@ -43475,7 +43475,7 @@ msgstr ""
 msgid "is not allowed since the group is not top-level group."
 msgstr ""
 
-msgid "is not allowed."
+msgid "is not allowed. Please use your regular email address."
 msgstr ""
 
 msgid "is not allowed. We do not currently support project-level iterations"

spec/models/user_spec.rb

@@ -498,7 +498,7 @@ RSpec.describe User do
     end
 
     describe 'email' do
-      let(:expected_error) { _('is not allowed for sign-up. Check with your administrator.') }
+      let(:expected_error) { _('is not allowed for sign-up. Please use your regular email address. Check with your administrator.') }
 
       context 'when no signup domains allowed' do
         before do
@@ -550,7 +550,7 @@ RSpec.describe User do
           user = create(:user, email: "info@test.example.com")
 
           expect { user.update!(email: "test@notexample.com") }
-            .to raise_error(StandardError, 'Validation failed: Email is not allowed. Check with your administrator.')
+            .to raise_error(StandardError, 'Validation failed: Email is not allowed. Please use your regular email address. Check with your administrator.')
         end
       end
 
@@ -623,7 +623,7 @@ RSpec.describe User do
             user = create(:user, email: 'info@test.com')
 
             expect { user.update!(email: 'info@example.com') }
-              .to raise_error(StandardError, 'Validation failed: Email is not allowed. Check with your administrator.')
+              .to raise_error(StandardError, 'Validation failed: Email is not allowed. Please use your regular email address. Check with your administrator.')
           end
         end
 
@@ -700,7 +700,7 @@ RSpec.describe User do
             user = create(:user, email: 'info@test.com')
 
             expect { user.update!(email: 'info@gitlab.com') }
-              .to raise_error(StandardError, 'Validation failed: Email is not allowed. Check with your administrator.')
+              .to raise_error(StandardError, 'Validation failed: Email is not allowed. Please use your regular email address. Check with your administrator.')
           end
 
           it 'does accept a valid email address' do