diff --git a/app/mailers/abuse_report_mailer.rb b/app/mailers/abuse_report_mailer.rb
index fe5f68ba3d502dc97ca2f402bec786d352d90505..e032f568913f185047013e9e24ed2b46a837ed89 100644
--- a/app/mailers/abuse_report_mailer.rb
+++ b/app/mailers/abuse_report_mailer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class AbuseReportMailer < BaseMailer
   def notify(abuse_report_id)
     return unless deliverable?
diff --git a/app/mailers/base_mailer.rb b/app/mailers/base_mailer.rb
index 654468bc7fe478d014dc9d63149a3b2dd026e895..5fd209c4761d2dd38506cc714d48628c2609365d 100644
--- a/app/mailers/base_mailer.rb
+++ b/app/mailers/base_mailer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class BaseMailer < ActionMailer::Base
   around_action :render_with_default_locale
 
diff --git a/app/mailers/devise_mailer.rb b/app/mailers/devise_mailer.rb
index 962570a0efd1ec211d4f4c3348fbb6823570c0b1..7aa75ee30e625ed16ecd6cbeab9d3f84a664df3e 100644
--- a/app/mailers/devise_mailer.rb
+++ b/app/mailers/devise_mailer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class DeviseMailer < Devise::Mailer
   default from: "#{Gitlab.config.gitlab.email_display_name} <#{Gitlab.config.gitlab.email_from}>"
   default reply_to: Gitlab.config.gitlab.email_reply_to
@@ -9,8 +11,9 @@ class DeviseMailer < Devise::Mailer
   protected
 
   def subject_for(key)
-    subject = super
-    subject << " | #{Gitlab.config.gitlab.email_subject_suffix}" if Gitlab.config.gitlab.email_subject_suffix.present?
-    subject
+    subject = [super]
+    subject << Gitlab.config.gitlab.email_subject_suffix if Gitlab.config.gitlab.email_subject_suffix.present?
+
+    subject.join(' | ')
   end
 end
diff --git a/app/mailers/email_rejection_mailer.rb b/app/mailers/email_rejection_mailer.rb
index 76db31a4c4546e1f75c8739afd49f91e4b2b92c0..45fc5a6c3837ce3e6dddf3b2354842744f9c2b6c 100644
--- a/app/mailers/email_rejection_mailer.rb
+++ b/app/mailers/email_rejection_mailer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class EmailRejectionMailer < BaseMailer
   def rejection(reason, original_raw, can_retry = false)
     @reason = reason
diff --git a/app/mailers/emails/issues.rb b/app/mailers/emails/issues.rb
index 392cc0bee03dcf8fc69595977d55ef2ce83dabd0..c8b1ab5033a99df5f7dd8be52ddc35577b7a3e84 100644
--- a/app/mailers/emails/issues.rb
+++ b/app/mailers/emails/issues.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Emails
   module Issues
     def new_issue_email(recipient_id, issue_id, reason = nil)
diff --git a/app/mailers/emails/members.rb b/app/mailers/emails/members.rb
index 75cf56a51f239d51f53523cc9609ca88257a24f7..91dfdf589824a71981f0f22e91ece7674375e44e 100644
--- a/app/mailers/emails/members.rb
+++ b/app/mailers/emails/members.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Emails
   module Members
     extend ActiveSupport::Concern
diff --git a/app/mailers/emails/merge_requests.rb b/app/mailers/emails/merge_requests.rb
index 70509e9066d21124e6b057a2a2a03e605a226f3b..70f65d4e58d2bd5652667443462ba3fdde4cb436 100644
--- a/app/mailers/emails/merge_requests.rb
+++ b/app/mailers/emails/merge_requests.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Emails
   module MergeRequests
     def new_merge_request_email(recipient_id, merge_request_id, reason = nil)
diff --git a/app/mailers/emails/notes.rb b/app/mailers/emails/notes.rb
index d9a6fe2a41ef526a1da58762ee28550596e57eef..d3284e90568ca63a3dc75f15f20d1cd03975fbcf 100644
--- a/app/mailers/emails/notes.rb
+++ b/app/mailers/emails/notes.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Emails
   module Notes
     def note_commit_email(recipient_id, note_id)
diff --git a/app/mailers/emails/pages_domains.rb b/app/mailers/emails/pages_domains.rb
index 0027dfdc36b1d9bc4b440e15f13b19d0f22f47bc..ce449237ef642b97675b3c0add995fb369a78e26 100644
--- a/app/mailers/emails/pages_domains.rb
+++ b/app/mailers/emails/pages_domains.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Emails
   module PagesDomains
     def pages_domain_enabled_email(domain, recipient)
diff --git a/app/mailers/emails/pipelines.rb b/app/mailers/emails/pipelines.rb
index f9f45ab987b39c08429f59eed603ec752e6521d2..31e183640adc4022db38ae350fe341534d992fc8 100644
--- a/app/mailers/emails/pipelines.rb
+++ b/app/mailers/emails/pipelines.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Emails
   module Pipelines
     def pipeline_success_email(pipeline, recipients)
@@ -39,10 +41,10 @@ module Emails
     end
 
     def pipeline_subject(status)
-      commit = @pipeline.short_sha
-      commit << " in #{@merge_request.to_reference}" if @merge_request
+      commit = [@pipeline.short_sha]
+      commit << "in #{@merge_request.to_reference}" if @merge_request
 
-      subject("Pipeline ##{@pipeline.id} has #{status} for #{@pipeline.ref}", commit)
+      subject("Pipeline ##{@pipeline.id} has #{status} for #{@pipeline.ref}", commit.join(' '))
     end
   end
 end
diff --git a/app/mailers/emails/profile.rb b/app/mailers/emails/profile.rb
index 4f5edeb9bda172872def7fd8a6d7396ff77a5e62..40d7b9ccd7a83c01ae2733f1b4c5c72ca4182cfc 100644
--- a/app/mailers/emails/profile.rb
+++ b/app/mailers/emails/profile.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Emails
   module Profile
     def new_user_email(user_id, token = nil)
diff --git a/app/mailers/emails/projects.rb b/app/mailers/emails/projects.rb
index 761d873c01c48b512a7ce4f95de423715fc729ae..d7e6c2ba7b2657b1e677f9c05631f68c058f560a 100644
--- a/app/mailers/emails/projects.rb
+++ b/app/mailers/emails/projects.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 module Emails
   module Projects
     def project_was_moved_email(project_id, user_id, old_path_with_namespace)
diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index 0e1e39501f51c87b1c094b249cd1619ebedd59aa..f4eeb85270e19890292ad647510824e9bfa2a78f 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class Notify < BaseMailer
   include ActionDispatch::Routing::PolymorphicRoutes
   include GitlabRoutingHelper
@@ -92,12 +94,14 @@ class Notify < BaseMailer
   #   >> subject('Lorem ipsum', 'Dolor sit amet')
   #   => "Lorem ipsum | Dolor sit amet"
   def subject(*extra)
-    subject = ""
-    subject << "#{@project.name} | " if @project
-    subject << "#{@group.name} | " if @group
-    subject << extra.join(' | ') if extra.present?
-    subject << " | #{Gitlab.config.gitlab.email_subject_suffix}" if Gitlab.config.gitlab.email_subject_suffix.present?
-    subject
+    subject = []
+
+    subject << @project.name if @project
+    subject << @group.name if @group
+    subject.concat(extra) if extra.present?
+    subject << Gitlab.config.gitlab.email_subject_suffix if Gitlab.config.gitlab.email_subject_suffix.present?
+
+    subject.join(' | ')
   end
 
   # Return a string suitable for inclusion in the 'Message-Id' mail header.
diff --git a/app/mailers/previews/devise_mailer_preview.rb b/app/mailers/previews/devise_mailer_preview.rb
index d6588efc4860ab2c9675da45f06b69eb29f08a68..3b9ef0d3ac04fb833af2f3aac5e79f7cdbcd04ad 100644
--- a/app/mailers/previews/devise_mailer_preview.rb
+++ b/app/mailers/previews/devise_mailer_preview.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class DeviseMailerPreview < ActionMailer::Preview
   def confirmation_instructions_for_signup
     DeviseMailer.confirmation_instructions(unsaved_user, 'faketoken', {})
diff --git a/app/mailers/previews/email_rejection_mailer_preview.rb b/app/mailers/previews/email_rejection_mailer_preview.rb
index 639e8471232f0704e5ff6fff25ab2b9ec3b00a3d..402066151ef074fc793aa2fa3f9af8fd0eac2dd1 100644
--- a/app/mailers/previews/email_rejection_mailer_preview.rb
+++ b/app/mailers/previews/email_rejection_mailer_preview.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class EmailRejectionMailerPreview < ActionMailer::Preview
   def rejection
     EmailRejectionMailer.rejection("some rejection reason", "From: someone@example.com\nraw email here").message
diff --git a/app/mailers/previews/notify_preview.rb b/app/mailers/previews/notify_preview.rb
index 3615cde802641790c2b99fac4280f062dec94da5..df470930e9e6df2bbfd736e7b58826910905989f 100644
--- a/app/mailers/previews/notify_preview.rb
+++ b/app/mailers/previews/notify_preview.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class NotifyPreview < ActionMailer::Preview
   def note_merge_request_email_for_individual_note
     note_email(:note_merge_request_email) do
diff --git a/app/mailers/previews/repository_check_mailer_preview.rb b/app/mailers/previews/repository_check_mailer_preview.rb
index 19d4eab18054dc3cddf62a2ac1d5fde7abbdd9b1..834d7594719545504102e7bb4b88ddd3b59795e8 100644
--- a/app/mailers/previews/repository_check_mailer_preview.rb
+++ b/app/mailers/previews/repository_check_mailer_preview.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class RepositoryCheckMailerPreview < ActionMailer::Preview
   def notify
     RepositoryCheckMailer.notify(3).message
diff --git a/app/mailers/repository_check_mailer.rb b/app/mailers/repository_check_mailer.rb
index 22a9f5da646fe3ec64c9929b1bae41f52c6121a2..4bcf371cfc039404978efe836f9d14ff19e6b5fd 100644
--- a/app/mailers/repository_check_mailer.rb
+++ b/app/mailers/repository_check_mailer.rb
@@ -1,3 +1,5 @@
+# frozen_string_literal: true
+
 class RepositoryCheckMailer < BaseMailer
   def notify(failed_count)
     @message =
diff --git a/changelogs/unreleased/frozen-string-enable-app-mailers.yml b/changelogs/unreleased/frozen-string-enable-app-mailers.yml
new file mode 100644
index 0000000000000000000000000000000000000000..2cd247ca76cdd251384870a00e2d0e5225032d13
--- /dev/null
+++ b/changelogs/unreleased/frozen-string-enable-app-mailers.yml
@@ -0,0 +1,5 @@
+---
+title: Enable frozen in app/mailers/**/*.rb
+merge_request: 21147
+author: gfyoung
+type: performance