[Exim] Wildcard MX and Ignoring MX

In my current MTA (PMDF) I am using a wildcard MX record to intercept
(to virus scan etc...). I then switch to another channel to perform
another DNS lookup, ignoring all MX records. If the recipient hostname
resolves, I relay the message on the host specified by the A record. The
catch is that I do not necessarily know the name of the internal MTAs
for which I will be routing mail.

Can anyone give some guidance on how I would do this in EXIM?

TIA
Ed...