Once you are on your iPhone home screen do the following:

1. Go to Settings and select “Mail, Contacts, Calendar” option.
2. Select your IMAP account and choose the account ([email protected])
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.