I've also run a mail server for a long time. Yahoo and Hotmail are probably the most annoying, Yahoo because you can't do anything about it and Hotmail for bloc

I've also run a mail server for a long time. Yahoo and Hotmail are probably the most annoying, Yahoo because you can't do anything about it and Hotmail for blocking entire IP-ranges. I don't blame them though, preventing spam is very hard. Spammers always have spf,dkim,dmarc,reverse dns,signing, etc meanwhile many legitimate senders has none. I'ts a hard problem to solve. I've tried things like increasing the response time so the sender have to sit and wait, or used spam lists, but there are too many false positives. One easy solution is to make it illegal to send spam. Since my country made sending unsolicited e-mail an offense, spam decreased a lot. The second problem is hacked servers. Unless you are running a mail server, always block port 25 in the firewall. So at least the hackers wont be able to send spam on your behalf. Many ISP's already block port 25.