[Exim] restricting access to local aliases

Because this seems to be a useful thing to ask exim to do, I'm
confidently expecting a RTFM style response. But I really have
trawled through the FAQ and example config files without finding
anything. So try to be gentle to an inexperienced exim admin.

Spam that goes to a few users we can live with, but I'd like to
stop spam to dozens or hundreds of users at a time. What I want
is to restrict access to certain aliases to senders from our local
domain(s) only.

Having been through the Exim Specification, I envisage a director
just before our normal aliasfile ones. It will have verify_only
set and then some test for a non-local sender (either using
senders = !... or by testing the value of $sender_host_name).
Then I'll do an lsearch of a file that will alias to :fail:
those aliases that we only want local users to be able to send to.
Because we have receiver_verify set this rejection will happen during
the SMTP dialogue and I can set an appropriate failure message to
go with the 550 fail code.

But maybe someone has done this already and wouldn't mind sharing
their configuration file? Or can point out what I'm about to do
wrong above?



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I always wanted to be someone - I guess I should have been more specific