I bumped into this a few months ago with a client. To fix it —
1) Disable (temporarily) outbound email checking in antivirus software (you will turn this back on when you have confirmed a solution was found).
2) Delete (or move) the messages in the SENT box. Deleted and recreate the box. Compact and tried again. This might fix it. If it does not, check option #3 (which was the problem in my experience).
3) The owner in my case had made the folders to store email history as sub-folders in the inbox. Make new subfolders under local folders to do a gross sort, then moved all of the history folders into them. Run compress again.
Let us know if this works.