[Exim] Wish for future exim releases

Preambula: one of our mail hubs serving backup MX for
our clients and for some other ISP's clients. I need
a solution, where we provide backup only for hosts/networks,
specified by list.
Currently:

domainlist relay_to_domains = @mx_secondary

... and I cannot control domains, that points their backup MX
to us. Postfix have option for this, exim - not.

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VVS56-RIPE