Squid is a HTTP proxy and the email clients use POP3 or IMAP protocols. With default configuracion/environment I think you not be able to connect your MS Outlook through Squid.
Is needed some iptables rules to redirect traffic of those protocols to Squid.
Here are some tips, but I recommend you to read more because I am not expert in iptables:
First redirect all the traffic on port 80 to the proxy port, commonly 3128 like this:
iptables -t nat -A PREROUTING -p TCP –dport 80 -j REDIRECT –to-port 3128
Then add a rule for every port used (25 and 110) in nat table for POSTROUTING (for packages as they are about to go):
iptables -t nat -A POSTROUTING -p TCP –dport 25 -j MASQUERADE
iptables -t nat -A POSTROUTING -p TCP –dport 110 -j MASQUERADE
Hope it helps you.