Courtesy forwards have been a standard feature of e-mail systems about as long as there have been e-mail systems. A user moves or changes jobs or something, and rather than just closing the account, the mail system forwards all the mail to the user’s new address. Or a user with multiple addresses forwards them all to one place to be able to read all the mail together. Since forwarding is very cheap, it’s quite common for forwards to persist for many years.

Unfortunately, forwarding is yet another thing that spam has screwed up. If you just forward all the mail that arrives at a typical address, most of what you’ll be forwarding is spam. From the point of view of the system you’re forwarding to, you’re the one sending the spam, and they’re likely to block you.

Fortunately, there are some ways to mitigate the damage.

• Configure separate outbound IPs, use one for forwarded mail, one for locally generated stuff. That way, if the forwarding goes wonky, only forwarded mail gets blocked. All of your outbound IPs should have reasonable and matching A and PTR records in the DNS. (This last bit applies whether you’re forwarding or not.)
• Use all the usual conservative DNSBLs such as Spamhaus and Spamcop to hard block mail. Their error rate is low enough that you don’t have to worry about it.
• Run the mail through Spamassassin or something similar. Some ISPs allow you to put an X-Spam-Flag: Yes header on the mail and send it along to be dropped in the user’s spam folder. Most don’t, in which case you should throw it away or put it in a local mail account they can check occasionally.
• Point out to your forwarding users that it is cheap and easy to configure modern mail programs to check multiple accounts. Rather than forwarding the mail somewhere else, deliver it to a local mailbox and have them configure their mail program to pick it up along with mail from other accounts. Most webmail systems now have a way to import mail via POP (the same way desktop mail programs pick it up). Even if there’s a lot of spam mixed in, it won’t affect your system’s outgoing reputation since you didn’t send it to their incoming mail server.
• If you control your IP ranges, sign up for feedback loops at the ISPs you forward to, so you can see what people are complaining about.

If you do these, you should get mail through to most places. The local pickup trick will get mail through to anywhere.