howto retrieve email using email client on LAN through squid proxy server

Dear friends,

I’m new user on linux. The place where I work require internet access for several computers so I set the lan to access internet through proxy server, & I’m using squid for it & it works. But the problem is my lan users cannot retrieve their email by using email client software like thunderbird or outlook. What configuration on the squid I should change so it can forward email using pop3 protocol, thanks !

Regards,

Junaidi