[thatsaguy]

From my perspective (~500 employee mail server), greylisting had a much larger impact at the time, thanks to the spambots/viruses attempting direct connection to mail servers. Extremely effective, zero false positives, much lighter on resources. I did use both, of course, so that I could keep a record of how effective the systems were.

Today the situation has flipped. Most of the spam we get is coming from authoritative servers (ie: gmail, yahoo, etc), making stuff like SPF/DKIM/etc next to worthless from a spam perspective (it's still marginally useful for forgeries), while bayes (or in general, trainable) filters are essentially the only thing that can differentiate it reliably.

With a modern setup, you can basically next to zero spam and no false positives. In fact, honest email marketing (ie: mailing lists you've actually subscribed to) are from my experience the only thing that throws these filters off.

[_urga]

Thanks, one thing we also found is that spammers tend to be poor at RFC standards, in a way that Gmail etc. will have no problem with, but which are obviously broken.

For example, we use our own https://github.com/ronomon/mime to detect and reject email which has missing multi-parts (no terminating boundary delimiter). All of this has been spam so far, and we are yet to see a false positive. I don't think SpamAssassin has a rule for this (yet)?

Another example is illegal header characters, which are almost always spam, with a handful of false positives (usually machine-generated).

[readingnews]

That is an interesting approach. Care to let us know how you go from https://github.com/ronomon/mime to some kind of SMTP server plugin (like for postfix for example)?

[_urga]

Thanks, you might find Haraka to be easiest since it's already Javascript.

Postfix may require a process callout, you might need to write a milter.

[voidmain0001]

I agree that greylisting was the cat's meow back then. I setup a VM running CentOS with Postfix and Postgrey as a MTA for our "work" email server and the result was a massive reduction in spam.