To answer the question:
man 5 postconf
sender_dependent_relayhost_maps (default: empty)
...
This information is overruled with relay_transport,
sender_dependent_default_transport_maps, default_transport and
with the transport(5) table.
So that is no good. But there is a solution:
You can override everything with the "FILTER transport:destination"
directive. With Postfix 2.7 and later, you can omit the destination,
so that Postfix will use the recipient domain as destination.
man 5 access
Note 3: the purpose of the FILTER command is to override message
routing. To override the recipient's transport but not the
next-hop destination, specify an empty filter destination
(Postfix 2.7 and later), or specify a transport:destination
that delivers through a different Postfix instance (Postfix 2.6
and earlier). Other options are using the recipient-dependent
transport_maps or the sender-dependent
sender_dependent_default_transport_maps features.
The FILTER action takes effect for all recipients of a message.
If a message has multiple FILTER actions, only the last one will
take effect.
Wietse
