Check against "Auto-Submitted: no" instead

This would be much more accurate. We assume this is an
auto-generated email if such header is provided, and
the value is not "no". It could also be: "auto-generated",
"auto-replied", or other values from extension. It seems
that only "no" could mean that this is sent by a human.

See: https://tools.ietf.org/html/rfc3834

lib/gitlab/email/receiver.rb

@@ -27,8 +27,7 @@ module Gitlab
         mail = build_mail
-        raise AutoGeneratedEmailError if
-          mail.header.to_s =~ /auto-(generated|replied)/
+        ignore_auto_submitted!(mail)
         mail_key = extract_mail_key(mail)
         handler = Handler.for(mail, mail_key)
@@ -91,6 +90,16 @@ module Gitlab
           break key if key
         end
       end
+
+      def ignore_auto_submitted!(mail)
+        # Mail::Header#[] is case-insensitive
+        auto_submitted = mail.header['Auto-Submitted']&.value
+
+        # Mail::Field#value would strip leading and trailing whitespace
+        raise AutoGeneratedEmailError if
+          # See also https://tools.ietf.org/html/rfc3834
+          auto_submitted && auto_submitted != 'no'
+      end
     end
   end
 end