Stopping Spammers Who Exploit Secondary MX?

drteknikal asks: "I'm the admin for a small law firm. We use our ISP as our secondary mx. We are receiving spam from our secondary mx even when our primary has been continually available. We suspect that spammers are routing to our secondary MX to bypass the DNS-based spam filtering on our primary. After examining some of the traffic, our ISP agrees. Neither of us sees an immediate solution, given the purpose and function of secondary MX. They already restrict relaying to hosts on their network. Has anyone else seen this? Does anyone have suggestions on how an ISP could secure their mail exchangers without interfering with the functionality required to function as secondary MX for an external domain?"

Comments

Spam filtering on their end...?

[greenhide]

Is there any reason why the ISP can't set up some sort of SPAM filtering on their end?

Also, why not just set up the ISP's server as a backup server only? That way, you access your e-mail through the main server, which would make the mail go through your SPAM rules, right?

Same thing here

[beat.bolli]

I observe the same thing here. 90%+ of the mails that reach us through the backup MX are spam. Our problem is that we have to get this mail with a POP account, thus bypassing the normal spam check.

One solution would be to let the backup forward the mails normally to the primary MX, but our ISP can't do this; once the mail is in the POP account, it's gone from the mail queue...

How about a third MX?

[Neon Spiral Injector]

I noticed the same thing. While both of the main servers had the same level of spam filtering, I found it odd that the secondary was seeing a lot more spam. So as an experiment I added a thrid MX, the first two are of priority 0 and 10. The third I made 100 (not that it really matters). On this third server I set up even stricter anti-spam rules. The amount of spam fell of very quickly after that. The spammers would go for the trap. I would say that over 90% of the e-mail to that server is spam. I have almost considered just making it a black hole. But it does see a little valid traffic, and there is a chance that both of our main servers could be offline at the same time.

Simple fix

[geirt]

Add your main server to DNS as the MX with both the highest and lowest priority.

eg:
mail.myserver.net pri 10
mailbackup.myisp.com pri 20
mail.myserver.net pri 30

Works perfect for now. But some day the spammers will adopt to this too ....

Use a local 3rd MX that feeds spamd

[petard]

They mostly seem to be going for the lowest MX on the list, so simply:

1. Obtain an additional IP address for your primary MX box.
2. Configure that as your tertiary MX, with, say, a priority of 100
3. Configure the MTA on that box to feed your Bayesian filter with all the spam you'll get, then delete it without transferring to your inbox :-)

The critical thing is that you need to be certain that this tertiary MX is only up when your primary MX is also up. No legitimate mail software will use an MX with a priority of 100 when it can connect to an MX with a priority of 1, so you can be certain that anything the 3rd MX gets is spam. If they're on different boxes, however, a simultaneous outage of the first 2 could cause you to accept then dump legitimate messages.