fixing mail problem: The IMAP command “APPEND” (to Sent Messages) failed with server error: Error in IMAP command received by server

I recently discovered an issue with a coworker where her Mail sent folder was automatically generating a single sent mail message over and over and over to the point where there was over a hundred messages of the same message being generated for no explainable reason.   Mail kept throwing the following error with each message generation:

The IMAP command “APPEND” (to Sent Messages) failed with server error: Error in IMAP command received by server..


here's the steps i had to follow to fix the issue.



Here are some more detailed ones:
1) Make sure that 'Mail' is fully quit (quit it)
1) Open 'Applications' in 'Finder'
2) Open 'Utilities'
3) Finally double click on 'Terminal'
For the following parts, do exactly as I say...
4) Type: 'cd Library/Mail' then press return (do not type the '
characters on this or following instructions)
5) Type: 'cd IMAP<youremailaddress>@[email address]' nthen
press return
6) Type: 'rm -rf .OfflineCache/' and press return
You can now quit terminal and restart Mail