Verify IMAP and submission port

These hints were taken from Support Plesk

IMAP via SSL using port 993

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