The POP3 Proxy only handles incoming emails. Outgoing emails go via your Firewall rules. The SMTP Proxy can be used for externally-hosted email for your proprietary domain, but not for Gmail, Yahoo, etc.
Cheers - Bob
Sophos UTM Community Moderator Sophos Certified Architect - UTM Sophos Certified Engineer - XG Gold Solution Partner since 2005
You might be able to use the SMTP Proxy by configuring a smart host similar to what jcherian did:
You also will want to configure the 'Relaying' tab to put "Internal (Network)" into 'Allowed hosts/networks' and to select 'Scan relayed (outgoing) messages'.
Cheers - Bob
Sophos UTM Community Moderator Sophos Certified Architect - UTM Sophos Certified Engineer - XG Gold Solution Partner since 2005