If you need to archive a copy of all inbound messages before Cerb processes them, you can add this line to the top of your framework.config.php file:

define('DEVELOPMENT_ARCHIVE_PARSER_MSGSOURCE', true);

Messages are archived in the filesystem under storage/mail/archive/<year>/<month>/<day>.

Cerb doesn't do anything with these files after they're written, so you can move or delete them as needed.