pkernstock
It’s not the existence of @ altogether, rather it’s the existence of multiple @ characters. See here:
As you can see, this is a particularly weighty score. SpamAssassin claims that messages with a score of -5 or greater are considered spam, but this is not the same case for all spam filters. Tests of a few different filters yield wildly differing results, but the common denominator is the extra @ symbol.
To clarify, your message-id wouldn’t be flagged - one @ symbol is expected.
Our environment is pretty small. Mailcow is run off a single machine (actually an old laptop, but it serves a purpose and runs smoothly). We have appropriate SPF, rDNS, etcetera. We use our email primarily for B2B communication, but we don’t advertise through it. Unsolicited mail is not generally part of our remit, most of our business is generated through advertisement of our product line through websites like reddit. The consultancy work that we do relies heavily on email communication, however, which is where this could have a really rather significant impact. It looks fairly poor to have a CEO’s mail end up in the spam bin, where most of the rest of our mail does not.