Is there a way to relay spam filter error message to the user? Like the one below:
CONFIG_TEXT: 451 Your account has been locked. Please contact your administrator. (in reply to end of DATA command)
This is not a built-in functionality in Plesk at the moment. However, it is possible to create the suggestion on the UserVoice portal. The top-ranked suggestions are going to be implemented in the future Plesk versions.