Unable to send messages from a Plesk server to external email addresses: security: example.com did not encrypt this message

Symptoms

• Emails sent to an external account, such as Gmail, are not delivered and the following message is being displayed:

  security: example.com did not encrypt this message

• Mail is not delivered with the following message in /var/log/mailllog:

  status=deferred (delivery temporarily suspended: TLS is required, but was not offered by host 127.0.0.1[127.0.0.1])

Cause

Incorrect Postfix configuration.

• Parameter smtp_tls_security_level = may is incorrect or missing in the /etc/postfix/main.cf file:

  # grep smtp_tls_security_level /etc/postfix/main.cf

Resolution

1. Connect to the server via SSH

2. Edit the file /etc/postfix/main.cf and set the parameter to a correct value:

   # grep smtp_tls_security_level /etc/postfix/main.cf
   smtp_tls_security_level = may

3. Restart Postfix service:

   # service postfix restart