mailq
less /var/log/mail.log
postsuper -d ALL
postsuper -d <ID>
systemctl restart postfix

apt update
apt install postfix
apt install postfix libsasl2-modules          # relay mit smtp auth

/etc/postfix/main.cf

sender_canonical_maps = hash:/etc/postfix/sender_canonical

/etc/postfix/sender_canonical

root    root@host.example.org

chmod 600 /etc/postfix/sender_canonical
postmap /etc/postfix/sender_canonical

If not already installed:

apt install libsasl2-modules

/etc/postfix/main.cf

relayhost = [smtp.example.com]:587
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/relay_pass
smtp_sasl_security_options = noanonymous
smtp_use_tls=yes
smtp_tls_security_level = encrypt

/etc/postfix/relay_pass

[smtp.example.com]:587 user@example.com:password

chmod 600 /etc/postfix/relay_pass
postmap /etc/postfix/relay_pass

apt install mailutils

echo "Test" | mail -s "Test mx01" test@example.com

openssl s_client -connect mx01.example.org:587 -starttls smtp