Hi, I use the squid proxy server too.
The ip for the ethernet connection (for your clients) on the server will be
listed in /etc/squid/squid.conf
http_port 192.168.0.2:3128 transparent
On my system this is eth0,
if I change the config (ip) for eth0 it will affect squid,
unless I change squid as well.
What ports does thunderbird require?
In squid.conf use acl Safe_ports
acl Safe_ports port 80 # http
You may need to change the port settings in your firewall too.
I use an iptable script, Forwarding Ports, or maybe the defalt rule will do anyway.
ps, this is a good page...