override subject method in devise mailer

app/mailers/devise_mailer.rb

@@ -3,4 +3,12 @@ class DeviseMailer < Devise::Mailer
   default reply_to: Gitlab.config.gitlab.email_reply_to
 
   layout 'devise_mailer'
+
+  protected
+
+  def subject_for(key)
+    subject = super
+    subject << " | #{Gitlab.config.gitlab.email_subject_suffix}" if Gitlab.config.gitlab.email_subject_suffix.length > 0
+    subject
+  end
 end

spec/mailers/notify_spec.rb

@@ -831,6 +831,7 @@ describe Notify do
     let(:user) { create(:user, email: 'old-email@mail.com') }
 
     before do
+      stub_config_setting(email_subject_suffix: 'A Nice Suffix')
       perform_enqueued_jobs do
         user.email = "new-email@mail.com"
         user.save
@@ -847,7 +848,7 @@ describe Notify do
     end
 
     it 'has the correct subject' do
-      is_expected.to have_subject "Confirmation instructions"
+      is_expected.to have_subject /^Confirmation instructions/
     end
 
     it 'includes a link to the site' do