How to test SMTP if the good ol' telnet on port 25 method is gone:

export CA=/etc/postfix/ssl/cacerts.pem
export CERT=/etc/postfix/ssl/signed-cert.pem
export KEY=/etc/postfix/ssl/key-nopasswd.pem

openssl s_client -starttls smtp -CAfile $CA -cert $CERT -key $KEY -connect mailhost:25

original letter here