Unix Networks question

Here's the problem! :
Our dialup users can choose to retrieve their email using either pop3 or smtp.  If They decide to use pop3, we create a directoy on the server called
/var/spool/popmail/x1/x2
x1 is the first letter of their hostname
x2 is the FQDN e.g. bob.test.co.uk
so for this address we would create
/var/spool/popmail/b/bob.test.co.uk

If they decide to use SMTP to retrieve their mail, we don't create this directory.

When sendmail recieves email it has to make the following decision-
If for example the email address that sendmail has to deliver to is bob@test.domain.co.uk

If the directory /var/spool/popmail/t/test.domain.co.uk exists, use one delivery agent (Mpop) to write to that pop3
mailbox.
If the directory doesn't exist use Mddn to deliver it to the mailq.

We currently have an old version of sendmail that seems to have slightly modified code to enable this.
We now need to upgrade to sendmail 8.8.7 but we dont have the source code for the older version of sendmail and thus cannot get sendmail 8.8.7 to make this check.

Does anybody know

a)  a change that we could make to the sendmail.cf file to make the check (this would be the ideal solution)
b)  a change to the sendmail code that will make the check

Any help would be really appreciated :)

Cheers