I think I might be able to offer another explanation. People talk about the reputation of their individual IPs, but networks matter a lot too. DigitalOcean for

I think I might be able to offer another explanation. People talk about the reputation of their individual IPs, but networks matter a lot too. DigitalOcean for instance is an occasional source of spam; Linode is very rarely; 1and1 is a HUGE headache; and so on. I have some custom software in place that temporarily drops entire netblocks for abuse, and I did that to keep up with the spam filtering offered by larger services. With so many spammers now switching between cheap VPS hosts, it was getting impossible to do single-IP-only bans. Ever since the software was put into place the amount of spam actually making it into users mailboxes has fallen off a cliff. You have to be very very careful about which network you choose to host your mail server from.