This was relatively easy to implement, given that all the building
blocks were already lying around somewhere.

The updated transport manual page summarizes how relayhost etc.
play together.

Wietse

TRANSPORT(5) TRANSPORT(5)
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DESCRIPTION
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This mapping overrides the default transport:nexthop selection that is
built into Postfix:

local_transport (default: local:$myhostname)
...

virtual_transport (default: virtual:)
...

relay_transport (default: relay:)
This is the default for remote delivery to domains listed with
relay_domains. The relayhost and sender_relayhost_maps parame-
ters specify the default nexthop destination. When none is spec-
ified, the nexthop destination is the recipient domain.

default_transport (default: smtp:)
This is the default for remote delivery to other destinations.
The relayhost and sender_relayhost_maps parameters specify the
default nexthop destination. When none is specified, the nexthop
destination is the recipient domain.