Wouldn’t it be simpler to just use authenticated SMTP? There is a PHP plugin for that.
The PHP script will authenticate to MiaB and send the mail … I need to find the specifics for one of my users, so I will add them to this thread when I do.
Alas: Can I use PHPMailer with my MIAB mail server?
More useful info here: https://help.dreamhost.com/hc/en-us/articles/216140597-How-do-I-send-PHP-mail-via-SMTP-
Or here: https://www.lifewire.com/send-email-from-php-script-using-smtp-authentication-and-ssl-1171197