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 b

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 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).