Clients may see a message similar to the following...
Turn off and on the mail for the afflicted account...
1. - 5. Open Settings > Mail, Contacts, Calendars > pick the account > turn off mail >
then... turn it back on.
When you turn off the Mail, your account's mailbox will clear.
6. Go to the inbox and re-sync.
After the re-sync, and your messages appear, you should be able to move mail around to a new folder, including Trash.
You're done!