cd ~/Library/Mail/ cd IMAPfirstname.lastname@example.org rm -rf INBOX.imapmbox rm -rf etcUse the rm command to delete the contents. You may have to check what is in there using the ls command first. Next time you restart Mail.app, it will rebuild the folders under your IMAP folder. You can check the size before and after by running the du command:
du -h -d 1Enjoy!