IMAP via SSL using port 993:

 openssl s_client -showcerts -connect mail.example.com:993 -servername mail.example.com

POP3 via SSL using port 995:

 openssl s_client -showcerts -connect mail.example.com:995 -servername mail.example.com

SMTP via SSL using port 465:

 openssl s_client -showcerts -connect mail.example.com:465  -servername mail.example.com

SMTP via TLS/StartTLS using port 25 or 587

openssl s_client -starttls smtp -showcerts -connect mail.example.com:25 -servername mail.example.com