[Exim] Director condition question

Hi,

for historical reasons, I have an exim host that needs to be reachable
by SMTP from everywhere (MX for a couple of domains), but should only
accept messages for adresses in @example.com if they come in from a
given host (the MX for example.com; the host in question is not listed
in example.com's MX record list).

The following director does this:
|example_bounce:

| debug_print                  = "D: example_bounce for $local_part@$domain"
|  domains                      = example.com
|  condition                     = "${if eq {${lc:$sender_host_name}}{mailgate.example.com}{0}{1}}"
|  driver                       = smartuser
|  new_address                  = ":fail: Mail routing error. This host is not MX for $domain"

This scheme fails when an address for one of the other domains handled
locally on this host is aliased to an example.com address, since the
director fails this address because the message was not delivered via
mailgate.

Can I determine if an address has been generated locally via an alias?

Greetings
Marc

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