> That's easy enough, use sender_dependent_default_transport_maps,
> and a default transport that returns mail as undeliverable.
>
> /etc/postfix/main.cf:
> sender_dependent_default_transport_maps = hash:/etc/postfix/sender_relay
> default_transport = error:5.7.1 sender is not authorized
>
> /etc/postfix/sender_relay:
> # sender transport:nexthop
> us...@example.com smtp:relay-for-user-1.example
> us...@example.com smtp:relay-for-user-2.example
This could perhaps need some additional configuration to be able
to send bounces. Though if the relay has stringent per sender
logins (is really an MSA with sender<->login enforcement, not a
relay MTA), it may not be possible to send bounces. You may
want to configure working double-bounce delivery, just in case.
--
Viktor.
