So you have been going crazy trying in vain to delete messages from your iOS device only to get the incessant message, Unable to move to the trash. You have looked everywhere online for a solution. You may have even tried a few. Well here is the one that works.
Go To Settings [The Gear Icon].
Click Mail, Contacts, Calendars.
Click on your email account [Make sure this is set for all your IMAP accounts or problem will rear its ugly head at you.]
Click Account Info.
Click Advanced [At the very bottom. You will need to hide the keyboard to see the advanced button.]
Under the “Mailbox Behaviors” listing at the top, select “Drafts Mailbox”
In the new window, select the “Sent Mailbox” folder. From the “On the Server” listing in the lower portion of the window, select the “Sent” folder. There should be a check mark next to this folder. Do NOT check the “On My iPhone” “Sent” selection at the top.
Click the “Advanced” button on the top left to go back to the advanced page.
Click the “Deleted Mailbox”.
Click the folder named “Trash” folder under the “On the Server” portion. Not the iPhone Trash at the top.
Back Out to Complete the Changes
Click the “Advanced” button on the top left.
Click on “Account Info”.
Click “DONE” [do not click Cancel]
You may or may not need to reboot your iPhone or ipad depending on if it works right away.