Thank you rob0, I decided to go with your second suggestion:
> > 2. Postfix should *only* relay through one of the hosts in
> > sender_dependent_relayhost_maps (or deliver locally), and *never*
> > try to deliver directly to some external domain's MX.
That one could be done with "default_transport = error:not allowed"
and replacing sender_dependent_relayhost_maps with
sender_dependent_default_transport_maps.
This works as intended. After reading a bit more about error(8) and DSN
codes, I decided to go with a verbose error message I can recognize:
default_transport = error:5.1.8 no relay for this sender address
For reasons I don't fully understand, postfix' smtpd maps the return DSN
from 5.1.8 to 5.1.2, but that's not a concern to me.
Thanks again for your time,
Michael
