Note that this applies to listeners which otherwise appear to be functioning normally. That is, they are not throwing errors into the event viewer about authentication, or end-to-end access.
The event viewer may include an error similar to one of the following, from the FSMT service:
"Error occurred while deleting the email. Reason: Error writing to socket."
"Error occurred while reading and processing the email(s). Reason: Connection was lost.".
"Error occurred while reading and processing the email(s). Reason: The process cannot access the file 'C:\FRSEmail\Attachments\ITSMv7_journal\_lastmsg_5.eml' because it is being used by another process".
This is caused by a "stuck" message in the listener process. It either contains corrupt data, is too large, or contains an attachment that has similar problems.
Note that you may or may not see the message in the Before Transformation and/or After Transformation folders.
First, confirm in the listener properties, how many messages are being processed per scan. This number "n" will be necessary to know, because the problematic message of one of the "(n) least recent" messages in the inbox.
Note that you may need to alter your view, sorting, grouping, or filtering in your mail client to identify the offending message.
Once that message or group of messages is identified, move or delete them from the inbox, and then restart the FSMT service.