Blackberry doesn’t support updating the “Sent Items” folder as far as I can see. However, there are other solutions out there that give you “live access” to these folders so that it is updated whenever you send emails, even your drafts are synchronized. Take a look at www.openhand-mobile.com . This solution gives you live view of all folders in your exchange/domino/imap/etc. server. It also gives you access to your files wherever you are. A much better solution than Blackberry if you ask me.
NEW ANSWER: What version of BlackBerry Enterprise Server are you using? We are using Blackberry Enterprise Server 4.1sp4 with Exchange 2007sp1. I can receive the email w/attachment on BlackBerry (BB); open it on the BB; forward it to another personer on my BB; then go into to OL2003 client on my desktop and everything I did on the BB, including the SENT items are updated and the original email w/attachment is in my Inbox. Are you using an Internet email or what is your email server? P.S. there are settings to update the SENT ITEMS. After you go into the Messages Icon; Click for Menu and selection OPTIONS from Menu > Email settings > Save Copy In Set Folder = YES