Whenever you try to delete messages on your IPhone and you get the message unable to move to Trash you can do the following to correct that issue
- Click on Settings
- Click on Mail, Contacts, Calendars
- Under Accounts click on your Account
- Click on your Account again
- Scroll to the bottom and click on Advanced
- In the IMAP Path Prefix field insert INBOX
- Click on <Account in the upper left hand corner
- Click on Done
- Return to your Home Screen
- Reopen Mail and you should now be able to delete