diff --git a/CHANGELOG b/CHANGELOG
index baa4cebb61..a392254b53 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -1,5 +1,8 @@
 Please view this file on the master branch, on stable branches it's out of date.
 
+v 8.0.4
+  - Fix Message-ID header to be RFC 2111-compliant to prevent e-mails being dropped (Stan Hu)
+
 v 8.0.3
   - Fix URL shown in Slack notifications
   - Fix bug where projects would appear to be stuck in the forked import state (Stan Hu)
diff --git a/app/mailers/notify.rb b/app/mailers/notify.rb
index db2f9654e1..50a409c375 100644
--- a/app/mailers/notify.rb
+++ b/app/mailers/notify.rb
@@ -140,7 +140,7 @@ class Notify < BaseMailer
   #  * have a 'In-Reply-To' or 'References' header that references the original 'Message-ID'
   #
   def mail_answer_thread(model, headers = {})
-    headers['Message-ID'] = SecureRandom.hex
+    headers['Message-ID'] = "<#{SecureRandom.hex}@#{Gitlab.config.gitlab.host}>"
     headers['In-Reply-To'] = message_id(model)
     headers['References'] = message_id(model)
 
diff --git a/spec/mailers/notify_spec.rb b/spec/mailers/notify_spec.rb
index 2c97a521d9..f219af67ff 100644
--- a/spec/mailers/notify_spec.rb
+++ b/spec/mailers/notify_spec.rb
@@ -52,6 +52,7 @@ describe Notify do
     end
 
     it 'has headers that reference an existing thread' do
+      is_expected.to have_header 'Message-ID',  /<(.*)@#{Gitlab.config.gitlab.host}>/
       is_expected.to have_header 'References',  /<#{thread_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
       is_expected.to have_header 'In-Reply-To', /<#{thread_id_prefix}(.*)@#{Gitlab.config.gitlab.host}>/
       is_expected.to have_header 'X-GitLab-Project', /#{project.name}/