I followed the guide posted at http://www.zimbra.com/forums/showthread.php?t=1538 and got mail relaying through networksolutions. It works great; however, I recently got a smartphone and set up a filter to forward certain e-mails to my exchange account for notification purposes. The mail gets delivered, I get notified, and the sender gets a bounce (Undeliverable) message (???). Looking into the problem reveals that network solutions (even though they forward) sends a bounce message back saying that relaying is denied based on the "From:" address. This is presumably the address of the original sender.
It looks as if the mail filter "Forward" rule is more of a redirect. Does anyone have a suggestion on how to get around the bounce being sent back? Maybe a way of editing the headers on the forwarded message or something similar.