SSH tunelling is usually used to avoid firewall restriction or to ensure point-to-point encrypted communication.
For example, if you want to send email to smtp server “smtp.yourserver.com” on port 587, but your organization currently blocking smtp port 25 and 587, then you can benefit from SSH tunelling to avoid from being blocked.
To get around that, you need an intermediate server, fastssh.com currently provide SSH tunneling service with 7days trial account.
Simple SSH tunnelling command, if you’re using fastssh.com service.
ssh -f email@example.com -L 2000:smtp.yourserver.com:587 -N
So in your mail setting, you can safely put, SMTP Server = “127.0.0.1”, SMTP port = “2000” in your setting, in order to automagically connect to “smtp.yourserver.com” port 587 without firewall restriction.
Please refer here, for port forwarding in Microsoft Windows environment using PuTTY