Hacker News new | ask | show | jobs
by mike_hearn 1527 days ago
Most likely, activity on your IPs or IP blocks you can't see. Spam filters use a kind of 'neighbourhood reputation' as well as info about specific domains.

Unfortunately, the case where you have one good but very low volume sender surrounded on all sides by high volume spammers is very difficult to handle for spam filters, no matter how well designed they are. And, programmers who self host mail on cheap VPS systems are by far the most likely to encounter this problem.