When you add your mail account to your mail program(or App), it should automatically configure your Sent folder in this manner. However, in some cases, such as with Apple Mail, it will automatically use a folder called "Sent Messages" instead of "Sent", which is inconsistent with other mail apps (including our own webmail) which prefer simply "Sent", causing you to need to switch between server-side folders to see all of your sent messages (rather than look in just one spot).

For iOS (iPhone & iPad):

1. Launch the Settings app
2. Choose Mail Contacts and Calendars (or Mail)
3. Select the account with which you are having trouble syncing the Sent folder
4. Under IMAP tap on your Account.
5. Choose Advanced at the bottom
6. Under Mailbox Behaviours, tap on "Sent Mailbox"
7. Ensure that the folder called "Sent" is checked off under the "On the Server" section