I know I hated seeing my filed or sent email messages in my Messages application on my BlackBerry 8703. This tutorial lets one change that.
You can hide sent and filed email messages by first clicking on the Messages icon. Then go to Options > General > Hide Sent Messages and Hide Filed Messages. This can help with a mailbox that is too full or if you send large quantities of email messages.
By default, BlackBerry phones are set up so that Reconcile is only set to Desktop, not Desktop and Handheld. I always change this option since it allows our Exchange users to file email messages while on the road and have those changes be reflected in their Outlook/Exchange mailbox.