I recently discovered that my ISP's spam filter (which blocks spam without downloading it to my PC, and stores it in a folder on the server before it gets deleted automatically after an interval) was blocking valid emails as well as the rubbish. So it's worth logging into your email server on the web if you can, to check the spam folder before it gets deleted (assuming you have an auto-delete function on the server).

I run a few websites so I don't want to block all messages from unknown senders, because I get messages from strangers contacting me through the website. As I own various domains, I've found that the spammers' latest idea is sending spam to random names at these domains, e.g. xzqqy@mydomain.com. I put a stop to that by setting up a rule to receive all messages from the valid names, and bin the rest. It doesn't prevent spam to the valid addresses, but it does reduce the amount of time I have to spend manually deleting it.

Btw, I encoded my website email addresses in javascript, which seemed to work for a while, but the spammers seem to have worked out how to decode them, unfortunately. I'll have to come up with something more devious :-(