Can you check to see if SRS is enabled when forwarding? This is configured under Settings>Antispam>Options card. SRS will rewrite the return-path information accordingly for forward events like this. Otherwise I recommend checking to see if you're on the latest minor as well, as there were some known automated forwarding issues in the past.
Kyle Kerst
System/Network Administrator
SmarterTools Inc.
(877) 357-6278
www.smartertools.com