iOS5: iPhone, iPod Touch, iPad
- Launch the Settings app
- Choose Mail Contacts and Calendars
- Select the account with which you are having trouble syncing the Sent folder
- Under IMAP tap on your Account. (NOTE: If it says POP, then you have to delete the account and re-add it as an IMAP account).
- Choose Advanced at the bottom
- Under Mailbox Behaviours, tap on "Sent Mailbox"
- Ensure that the folder called "Sent" is checked under the "On the Server" section
- Press the home button to finish making changes