[Exim] setting hosts in directors

I'm sorry if I am missing something simple here, but I have searched the documention
and don't think I am.

I have just been reviewing an exim config file and thought there must be a simpler
way of doing part of it. At present I have a smartuser director which is generating
a new address and calling an smtp transport, which in turn looks up the host, in
simple terms:

director:
driver = smartuser
domains = <host-list>
no_rewrite
transport = yyy
new_address = ${local_part}@<lookup to get "host-x">

transport yyy:
driver = smtp
gethostbyname
hosts = <lookup "host-x" in a file to get "host">

i.e I want to do an smtp transfer to "host" with an RCPT TO: of "user@host-x".

The smartuser director knows "host" without doing a lookup so it would be simpler
if I could do "hosts = host" in the smartuser director. That would save the need to
maintain the file and have the lookup in the transport.

Unfortunately there is no "hosts" option for directors, which seems odd since there
is a "fallback_hosts" option.

What am I missing?

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