[exim] setting smtp_accept_max_per_host

I want to make the value of smtp_accept_max_per_host depend on the IP address
of the connecting host, but can't see a decent way of doing it.

Note that the documentation says it is important that the string expansion for
this must be fast because it is expanded in the master daemon.

I thought I would be able to use:

  hostlist specials = 1.2.3.4 : 1.2.3.5 : ... a total of 20 IP addresses
  smtp_accept_max_per_host = \
    ${if match_domain{$sender_host_address}{+specials}{5}{10}}

but that does not work because the documentation says 'Host lists are not
supported... This is because hosts have two identities: an name and an IP
address'.

It seems that what is needed is a 'match_ip'!

I do not want to use ia domain list because that would force a series of DNS
lookups in the master daemon.

Can anyone suggest a suitable way of doing this (I'm sure I must be missing
something simple here!)?

Phil.
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Phil Chambers (postmaster@???)
University of Exeter