If you receive an error similar to 'unable to move to trash folder' while deleting the email on any iOS-based device, you can do the following to fix this. This also applies to move an email to any other mail folder.
- Choose the Settings app on your home screen
- Choose Mail, Contacts, Calendars on the left
- Select your mail account in the Accounts list
- Tap the Account email address, then Advanced at the bottom of the page
- For the IMAP Path Prefix setting, change it to INBOX in all capitals
- Go back (click the Account arrow in the upper left corner), then choose Done twice (upper right corner).
If it does not take effect immediately, restart your iOS device. Hold down the power button, slide to power off, then press the power button again after the screen goes black.