It's not clear what causes this, but sometimes users encounter an issue moving mail to the trash on the iPhone or iPad. Here is an easy-to-follow step-by-step guide for fixing this issue -
http://www.netchimp.co.uk/webdesign/iphone-ipad-ipod-touch-tips/iphone-unable-to-move-message-to-trash-imap/
- 3 Users Found This Useful