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Hi,
I have two servers, both running Exim, that should both handle some
addresses from a single domain; i.e., foo@domain is handled by one of
the servers, while bar@domain is handled by the other. By fiddling with
transports and routers in a non-active configuration file, I have been
able to configure this; I have a set of routers that will deliver mail
locally if it is a local address, but will route it to the other host if
it is not. So far, so good.
What I'd like to see, though, is that recipient verification would also
work. I was thinking of doing this by using callout verification, but
there is a problem:
If one of the servers receives an RCPT for a local part that does not
exist on either server, it will run an SMTP session to the other host,
as described in the documentation for the callout verification. Since
this looks like a normal SMTP session, it will get through the same ACL
as any other connection; thus, the other server, which also sees that
there is no local delivery possible, will perform a callout to the first
server. There is a loop here.
My first idea was to add a 'hosts=' option to the ACL that does the
callout to avoid the loop that way, but that can't work; that hosts
option will then apply to /all/ connections from the other system, which
will in turn result in the systems refusing legitimate mail if the mail
was offered to the 'wrong' system.
This is not what I want it to do. What's necessary is that the callout
should only be performed if the connection we're dealing with does /not/
originate from the system we would perform a callout to in the first
place. I've been searching in the documentation for an option to
accomplish this, but either I've missed something, or this isn't
possible... any hints?
Thanks,
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