Outlook and Outlook Express do NOT keep anything on servers with RoadRunner when you SEND e-mail, they are NOT using Exchange servers
Emptying your sent items is not going to do anything for you. What you may want to check is whether you have "keep mail on server" checked, which keeps a copy of every e-mail on the server until a client that does not have that setting downloads it.
You can also just go into webmail and see what is in there, but the Outlook suggestion is ludicrous, that would only work if you use Outlook in a business environment with an Exchange server.