[exim] Exim with Mailscanner Cyrus IMAP

I am using Mailscanner with Exim v4.28 delivering to Cyrus IMAP.

The first Exim process leaves mail in a queue where Mailscanner
takes over and scans for spam/virus and then calls the second
Exim process.

Trouble is, mailnot destined to a valid mailbox or alias gets through
to the second process where I would really like it to be rejected at
the first. Various tools for preventing dictionay attacks also
seem to realy on knowing at the first process whether the intended
recipient is valid or not.

Any tips on how to get this done? Most info I have found is for
checking for valid local accounts (mbox) or for setups only using
single Exim processes with IMAP.

Just a pointer to the FM would be appreciated.

Thanks.