A few ISPs use a non-standard and not recommended system of encrypting connections to their SMTP mail servers. These generally are called 'SSL' connections and work on an alternate port other than the standard SMTP port 25. (The correct way to encrypt connections is to use a system called TLS which works on the standard SMTP port).
To use the non-standard method, in the SMTP Relay Servers box, type the SMTP relay server name, followed by :<alternate port number>, eg smtp.isp.com:465
Then, in the Session Encryption box at the bottom of the page, choose SSL