Altough I am n00b I think that you should either:
1. Set up a firewall with ip-masquerading and ip-forwarding on you'r server where you can control allowed hosts and so on (instead of proxy)
2. Set up a filter on the server from where you can get you'r mail e.g spambayes (of course this only works if you can get mail from within the server)
3. Weird squid-proxy-configuration!?
Dunno if this is the answers but at least I tryed :-)
I hope you solve it, cheers!