Test SSL/TLS connection

Protocol	Port
HTTPS	443
IMAPS	993
IMAP STARTTLS	143
POP3S	995
SMTPS	465
SMTP STARTTLS	587

Option	Description
-showcerts	Show full chain certificate
-servername <SNI>	Set TLS SNI
-verify_return_error	Stop if verification errors

openssl s_client [<options>] -connect <host>:<port>

Show TLS certificate

openssl s_client -connect <host>:<port> [-servername <SNI>] | openssl x509 [-noout] [-text]

openssl s_client -connect example.org:443 -servername example.org | openssl x509 -noout -text

Test SMTP / IMAP StartTLS

openssl s_client -starttls { smtp | imap | pop3 } -showcerts -connect <host>:<port> -servername <domain>

openssl s_client -starttls smtp -showcerts -connect example.org:587 -servername example.org