If you wish, you can set the IMTA_TCP_FLAG_RETENTION option in your imta_tailor file to, say 1, so that the old *.data-failed files get purged after a day. You can find these files in msg-<instance>/imta/ queue/<channel-name>/spool/. Note -- this directory will only exist if the MTA needed to write files to this directory. Many sites will never see this directory on their systems.