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Proper Ways to Dispose of Spam?
An anonymous reader asks: "My domain name is being stolen by spammers; they forge outgoing mail using my poor innocent domain name. First, I'd like to plead with mail server administrators out there: please REJECT spam and undeliverable mail. If you reject instead of bouncing then legitimate mail senders will still know there is a problem. Second, do you have any tips for dealing with a flood of spam bounces? Exim is pitching the bounces pretty quickly, but my server is still getting overwhelmed."
In the case of stolen sender addresses, SPF attempts to address this problem but has it been effective?
Two of my domain-names are in several spammer-tools and i was inundated by spam-bounces (and auto-replies). With SPF, i am down to one bounce every now and then.
It's great to set up your mail server to reject the mail up front. But many spammers know people are doing this, so they connect to backup MX, often the one with least priority. From what I've read, that's how spammers' mail blasting programs are written these days.
Are you running your own backup MX? Probably not. It's often a generic spooler your ISP lets you use for convenience. Even if you do, does your backup MX have all your rules in place, so it knows what to reject? No, I bet not. So this backup server accepts the mail without question, then passes it to the primary, and then it gets bounced.
We need to either have a way to give our backup MX our rulesets (which the people who run the backup servers understandably won't like), allow backup and primaries to just silently discard (which legitimate senders and receivers won't like), or, quite possibly, stop using backup MX entirely, and then if the primary goes down, the originating mail servers should do their normal pattern of retrying for 5 days, or whatever.
Large companies who need 100% instant availability of mail shouldn't be using backup MX anyway, (I've seen backup MX servers configured to hand off to primary hourly or even daily, not to mention those that hold until the primary asks for the mail) they should be using a ring of servers sharing primary preference. I'd expect the ruleset to be identical across the ring, thus allowing for instant rejection all the time.
You should not generate the bounce, a 5xx responce to an SMTP command is all your server should do. If it is a real mail server talking to yours it will generate the bounce for the user that is relaying through it (hopefully including the text of your 5xx reply).
Ahhh, I had one of those -yesterday-. We have SPF implemented, and it still doesn't work very well, alas.
I got a call from a sysadmin somewhere in nowheresville USA. The minute I picked up the phone, the guy started berating me, since I was destroying his domain, and it was all my fault, because I'm running Exchange and obviously I was infecting him with Winblows.
After I finally got things sorted out, I walked him through exactly how and why it wasn't our domain a'tall, which would have been obvious had he looked at the headers of any one of the thousands of emails he claimed he recieved. If he knows how to read any of them. When he realized he was wrong, he slammed the phone down midsentence.
Point of the story: SPF is great, proper mail server administration is great, but there will always be jerks who think they know what they're doing when they don't, and they're the bane of the whole system, more like a wolf in sheep's clothing than a known enemy.
ACs are modded -6. I don't read you, I don't mod you, I don't see you. Don't like it? Don't be a coward.
You have already failed the first rule of disposing of Spam:
1. For goodness sake, whatever you do, don't eat the stuff!
KFG
You start by rejecting outright email for non-existant email addresses. That gets rid of all bounces that come from addresses the spammers have made up. Then you look at the Received headers of the email that you supposedly sent and validate that it did indeed come from your IP and the header is of the form that your MTA generates. If not, somebody was impersonating you and you reject the bounce. See Stopping Backscatter Email.
The problem of invalid bounces drops dramatically if you set up your incoming server so that invalid addressees are rejected with a "User unknown" note at SMTP time. If you're using Sendmail with a virtual user table, this is as easy as adding the following at the end of the file
@example.com error:nouser 550 5.1.1 User unknown
It's important to do this on the server that accepts mail from the outside. If you have a setup with an antispam/virus gateway that then relays to an internal server, you need to make the gateway aware of the valid/invalid addresses.
By rejecting invalid senders in the SMTP transaction, you only get bounces from the few messages that forged an actual sender. In my experience, the addresses tend to look like ashawuiefgfyig@example.com, so most of the bounces will just disappear into the ether(net).
There is a Postfix backscatter HOWTO at http://www.postfix.org/BACKSCATTER_README.html
A DNS request is tiny compared to bouncing about bits of mail - if you can reject the message before even processing the body thanks to SPF you significantly reduce bandwidth consumption, much more than that spent on a DNS lookup, especially now there are so many image based spams floating about.