JimC is right

JimC is right. I get lots of spam addressed to my work e-mail address because someone outside the (Windows-free) office got infected with some kind of malware.

I'm lucky enough to have my own domain and my own MX, so I can issue individual prefixes to everyone who might ever need to e-mail me. This includes mailing lists. I've put my own rules in $HOME/spamassassin/user_prefs which add 20 points to mail addressed to address I use for the list, then subtract 20 points from mail actually originating from the list.

If it's true that MSN are rejecting e-mail from Mozilla clients, they may well be running afoul of European law regarding anti-competitive behaviour.