# cat /etc/squid/blocked_mailsites.reg
^http://win\.mail\.ru/
^http://mail\.yandex\.ru/

squid.conf:

acl mailservers url_regex "/etc/squid/blocked_mailsites.reg"
acl ws_mail src 192.168.1.34 192.168.1.202 192.168.1.149
http_access allow ws_mail mailservers
http_access deny mailservers