reject_unknown_recipient_domain,
out of -
smtpd_recipient_restrictions =
it now works.
How can I reinstate that setting for normal email, and bypass for fax?
Are you operating in a local private lan?
If so, I'd add "permit_mynetworks" as first argument of "smtpd_recipient_restrictions"
Also, teach postfix that fax.yourdomain.dom is something it should handle by adding it to "mydestination"