[Exim] How do I ...

I support a small business that has all it's email put into one pop account at an ISP. Every now
and them fetchmail goes & grabs the mail which is delivered locally to users like fred@???.
For this to work exim has to be told that it can do a local delivery of anything in foo.com (ie
it is in local_domains). The rest of the world delivers to the ISP as it advertises MX records
for foo.com that point to one of it's machines: blackbird.isp.co.uk -- this all works nicely.

The problem is that the ISP hosts a mailing list with an address of list@???. External people
(like me) can mail to the list without problems. My customer cannot because exim says
``list@???'' must be a local delivery and then can't find a local user `list' and so routes
it to an `unknownuser' rule that send it to the postmaster.

How do I pursuade email to recognise list@??? to be a special case that must be routed up to
the ISP ?

TIA

--
Alain Williams