1. Go to Settings and select Mail, Contacts, Calendar.
2. Select your IMAP account and choose the account (firstname.lastname@example.org).
3. Scroll down and select Advanced.
4. Change Mailbox Behaviors for Sent items (Choose Sent Mailbox and check Sent Items).
5. Go back to Advanced (top left), then back to Account (top left).
6. Choose Done to save.