Outlook has built in functionality to support this. When you find a message you need to forward, click forward or reply, then type what you need. Then choose Follow Up (it's on the first ribbon if you're using 2007, I think it's in the send options if you're using 2003). when you send the mail, the item will show up in your todo list.
An alternative approach is to manually copy the item from your sent items into a waiting for folder. Afterwards I flag the copied item and presto, it shows up in my todo bar. I personally do this and also attach a category to it that represents the project under which it's related.