If you receive the error message : Unable to move message The message cannot be moved to the mailbox trash when trying to delete a mail message on your IMAP mailbox, here is the fix :
1. Go to Settings -> Mail, Contacts, Calendars.
2. From accounts, select your mail account :
3. Click on your account again :
4. Scroll down and click on Advanced :
5. Click on Deleted Mailbox :
6. Click on Trash, under On My Iphone :
7. You won’t see anything happening, but now it should work.
Tested on iPhone4 running 4.3.5.