Undeliverable mail
Well, there are some mailhosts which are too lousy to handle spam with intelligent techniques so they've decided to do a IP-based filtering. This is against the free nature of email communication and I'm not willing to support such behaviour. If a server says "your IP is not good enough", I say "screw you!"...
If you happen to appear on the list below, please consider telling your mail provider or pick a different one...
...or you can always get in touch with me by other means (but you'll remain behind the barrier - other people may have problems delivering to you, too).