Why does it say "Unable to move message to trash" when I try to delete an email in iOS 8?
This happens when you don't have your Deleted Mailbox set to Trash. Here's how you set it properly in iOS 8 (and some previous iOS's):
1. Head to Settings -> Mail, Contacts, Calendar and tap on the email account you want to fix.
2. Tap on "Account."
3. Scroll down to "Advanced" and tap on it.
4. On this page, make sure "Deleted Mailbox" is set to Trash. If not, tap on it.
5. Now you'll see options to set "Deleted Mailbox" to. The top section is "On My iPhone" and has "Trash." Select this if you want trashed emails to only be in your iPhone Mail app's trash. This means that, although they will be deleted from your mail server inbox, they won't go to the server's trash (which means if you access your account on another computer/device, you won't be able to recover the email that you threw out; it is only in the trash on your iPhone. Alternatively, select "Trash" from the "On the Serve" section, so trashed emails will be available wherever you access your mail. I, for one, see no reason not to select server side trash.
6. Once you've selected you have to go back and hit "Done" to make sure your changes are saved. So tap "Advanced" in the top left, then "Account" in the top left, then "Done" in the top right.
Now you should be able to send email to the trash without incident.