Giuseppe Cassibba

Hi Jim,
maybe following article could be what you need:https://peppe8o.com/mailserver-on-raspberry-pi-zero-w-with-postfix-dove…
MTA agent (Postfix) part should be enought if you want only send e-mails. A different approach is using public SMTP services

This can be used both for local network and public network. In latter case, you need that your Internet Conenction has a public IP and map ports in your router (usually http/80 and-or https/443). Before this, you should consider enforcing security for your Raspberry PI to avoid external attacks and using a certificate.