POP (Post Office Protocol) is the most used protocol for receiving mail from a server to a MUA (Mail User Agent) for reading. It is specified in RFC1939. But the way it works (in most cases) is quite simple and doesn’t require reading the RFC, connect to port 110 (the standard port for POP3) and
The first thing to test is a TLS (aka SSL) connection. The stunnel program has special code for this, the command “stunnel -n smtp -c -r mail.example.com:25” will connect to the server via SMTP and negotiate SSL. If you use gnutls then the command “gnutls-cli -s mail.example.com -p 25” will connect to the server, allow