delay in /etc/mail/aliases

Take a look at Greylisting; a greylisting system looks at each incoming email's to field, from field and sending server, and, if it hasn't see that combination in the last 24 hours (configurable), it has the mail server send a 451 "try again later" until some amount of time has passed (I use 20 minutes), and then allows the mail to pass through.

I use Postfix+Postgrey, but there are Sendmail implementations, too:

http://www.greylisting.org/implementati … mail.shtml">http://www.greylisting.org/implementations/sendmail.shtml

Actually, I think the problem may be that when you forward, you cause the message to originate from an unauthorized server. For example, a message from joeuser@yahoo will seem to be originating from your server instead of Yahoo!, which Gmail might see as a forgery and flag as spam.

This problem is perhaps most closely associated with SPF:

http://www.openspf.org/FAQ/Forwarding

(see the note about "remailing").

Or it could even be the inverse problem. If the spam originates from a known bad IP, and then he resends it from a nominally trusted IP (Gmail's), then it could be less likely to be flagged.

I re-read the OP–I may have misunderstood. I wouldn't process (e.g. forward) anything without virus/spam-filtering it first. If you make yourself that front line, you'll have to handle it. :)