By default Apple Mail will not use the Sent folder on the server to place sent emails in when the server is configured for IMAP. If you wish to have your sent items stored on the server you will need to go through a short, simple process in order to have this done:

1. First you must select the Sent folder in the left side that is listed under the IMAP folders for the account. 
   
2. Next, click Mailbox from the menu bar. Go to Use This Mailbox For and in the box that opens up select Sent Mailbox. With this done Apple Mail will now use the Sent folder on the server to store sent emails rather than use the local Sent folder.