[exim] rotate inbound mail to multiple local accounts

Hi list,

I need to solve the following problem:

Inbound mail to a single account needs to be shared out between multiple
addresses. For example, if an alias "catchall@???", I want the
emails coming into this to be shared out between a number of local users
such as "user1@???", "user2@???", "user3@???". I
don't want mail copied between the accounts so the first mail that comes
in goes to "user1@..", then the next one to "user2@.." etc...etc...

Is there a built-in feature of exim that'll allow me to do this or am I
best off routing the mail to an external perl script to handle it? I
guess the script would keep a counter stored in a file so it knows where
the next email will go.

cheers,
Paul.