SMTP sends all mail generated by the ZFSSA, typically in response to alerts as configured. SMTP does not accept external mail; it only sends mail generated automatically by the ZFSSA itself.
By default, the SMTP service uses DNS (MX records) to determine where to send mail. If DNS is not configured for the ZFSSA's domain, or the destination domain for outgoing mail does not have DNS MX records set up properly, the ZFSSA can be configured to forward all mail through an outgoing mail server.