It's for Message-ID so it should be message_id

lib/gitlab/email/receiver.rb

@@ -60,7 +60,7 @@ module Gitlab
       def key_from_additional_headers
         Array(mail.references).find do |mail_id|
-          key = Gitlab::IncomingEmail.key_from_fallback_reply_mail_id(mail_id)
+          key = Gitlab::IncomingEmail.key_from_fallback_message_id(mail_id)
           break key if key
         end
       end

lib/gitlab/incoming_email.rb

 module Gitlab
   module IncomingEmail
     class << self
-      FALLBACK_REPLY_MAIL_ID_REGEX = /\Areply\-(.+)@#{Gitlab.config.gitlab.host}\Z/.freeze
+      FALLBACK_MESSAGE_ID_REGEX = /\Areply\-(.+)@#{Gitlab.config.gitlab.host}\Z/.freeze
       def enabled?
         config.enabled && config.address
@@ -21,8 +21,8 @@ module Gitlab
         match[1]
       end
-      def key_from_fallback_reply_mail_id(mail_id)
-        match = mail_id.match(FALLBACK_REPLY_MAIL_ID_REGEX)
+      def key_from_fallback_message_id(mail_id)
+        match = mail_id.match(FALLBACK_MESSAGE_ID_REGEX)
         return unless match
         match[1]

spec/lib/gitlab/incoming_email_spec.rb

@@ -43,9 +43,9 @@ describe Gitlab::IncomingEmail, lib: true do
     end
   end
-  context 'self.key_from_fallback_reply_mail_id' do
+  context 'self.key_from_fallback_message_id' do
     it 'returns reply key' do
-      expect(described_class.key_from_fallback_reply_mail_id('reply-key@localhost')).to eq('key')
+      expect(described_class.key_from_fallback_message_id('reply-key@localhost')).to eq('key')
     end
   end
 end