If you have noticed, when using outlook, a copy of your email is saved in Outlook's sent items folder, there is not one in the Sent folder on the Sitemason Mail IMAP server. Here is a how to on setting up such a rule in Outlook!

To set up the rule, go to Tools->Rules and Alerts (or Rules Wizard). Create a new blank rule, and then select Check messages after sending. Select a condition if you want to copy certain items to the IMAP folder, or leave it blank to copy every item you send. Now click Next.

Choose "Move a copy to the selected folder" as the action and select the Sent folder on your IMAP server, then click Next. Set up any exceptions you want and click Finish. Now all messages you send, or those meeting the conditions you selected, are copied to the IMAP server. If you have two or more email accounts and disable saving sent items, you'll need to create rules for the other accounts if you want to save the sent messages.

If you don't want a second copy locally, open Tools->Options->Preferences->Email Options and remove the check from "Save copies of messages in Sent Items folder". If you have more than one account, you should create a rule for each account, otherwise all sent items are moved to the IMAP server's Sent folder.