Yes, but milters are not much fun to write. Postfix has a way to use a sidecar MTA (like lamson/salmon) to filter. Effectively Postfix passes the mail to the ex

Yes, but milters are not much fun to write. Postfix has a way to use a sidecar MTA (like lamson/salmon) to filter. Effectively Postfix passes the mail to the external process via SMTP and then, if the process chooses to, it can re-inject the message into a queue in Postfix. I worked on a system years ago that did this for FBL reports.