How to use spam filtering on a VPS with only mail forwarding (aliases)?

Question

I have a VPS set up as MX for several of my domains. The e-mail for these domains are just aliases to other mail accounts (mostly Gmail) to forward incoming mail. Ofcourse this catches also much spam e-mail that is than rejected by Gmail (and my VPS becomse temporarily blocked)

2014-07-29 12:56:42 SMTP error from remote mail server after end of data: host alt3.gmail-smtp-in.l.google.com [74.125.142.27]: 421-4.7.0 [xxx.xxx.xxx.xxx 15] Our system has detected an unusual rate of\n421-4.7.0 unsolicited mail originating from your IP address. To protect our\n421-4.7.0 users from spam, mail sent from your IP address has been temporarily\n421-4.7.0 rate limited. Please visit\n421-4.7.0 http://www.google.com/mail/help/bulk_mail.html to review our Bulk\n421 4.7.0 Email Senders Guidelines. wc16si48135826icb.20 - gsmtp

I could of course install something like SpamAssassin on my VPS so it would filter out most spam. But as I'm not hosting the e-mail myself (e.g. providing IMAP) I can't give users a way to see if there were any false positives etc.

Is there a way to better manage spam on my VPS to avoid the constant blocking of Gmail, or is this something that is simply unavoidable?

Thanks!

Answer 1

You can't tell the receiving MX where you want your mail to go. I would set up SpamAssassin, set it to a threshold of 6 (instead of the default 5) and reject all mails above that threshold at SMTP level (with SA-Exim, for example). Then your mails either pass the spam check and get delivered or the sender (if it is a real mail server) gets a notification that the message could not be delivered because it looked spammy.