Sync sent email to the server on an iOS device (iPhone, iPad and iPod Touch)

iOS5: iPhone, iPod Touch, iPad

1. Launch the Settings app
2. Choose Mail Contacts and Calendars
3. Select the account with which you are having trouble syncing the Sent folder
4. 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).
5. Choose Advanced at the bottom
6. Under Mailbox Behaviours, tap on "Sent Mailbox"
7. Ensure that the folder called "Sent" is checked under the "On the Server" section
8. Press the home button to finish making changes

Source: http://clients.websavers.ca/whmcs/knowledgebase/115/Keeping-your-Sent-folder-in-sync-across-devices.html