When spam filters fail

Spam filters aren’t perfect. They sometimes catch mail they shouldn’t, although it happens less than some people think. They sometimes fail to catch mail they should.
One of the reason filters fail to catch mail they should is because some spammers invest a lot of time and energy in figuring out how to get past the filters. This is nothing new, 8 or 9 years ago I was in negotiations with a potential client. They told me they had people who started working at 5pm eastern. Their entire job was to craft mail that would get through Hotmail’s filters that day. As soon as they found a particular message that made it to the inbox, they’d blast to their list until the filters caught up. When the filters caught up, they’d start testing again. This went on all night or until the full list was sent.
Since then I’ve heard of a lot of other filter bypass techniques. Some spammers set up thousands of probe accounts at ISPs and would go through and “not spam” their mail to fool the filters (ISPs adapted). Some spammers set up thousands of IPs and rotate through them (ISPs adapted). Some spammers register new domains for every send (ISPs adapted). Some spammers used botnets (ISPs adapted)
I’m sure, even now, there are spammers who are creating new techniques to get through filters. And the ISPs will adapt.