[exim] Synchronising Authentication Between Servers

Hi Folks,
I am currently trying to add redundancy to our email setup, and
struggling a bit with authentication.
Essentially we have some accounts on a WHM server that store passwords
in the passwd/shadow format.
I have put together a script that collects the email addresses and
passwords and then puts them in the format:

email@???:$1$SgeC%ghWgjwRfksWFAKefgnl432GI74::

(for reference I have made up most of the characters in the password
string so it is not a valid password)

I have then copied this to a Centos 6 server as /etc/exim/passwd. The
server is running a fairly standard copy of Exim, but I don't seem to be
able to get password authentication to work.

I have dug up several guides, but they all seem to talk about using md5
passwords or PAM into the system accounts. None of these accounts are or
should be in the normal /etc/passwd.

Can anyone more knowledgable point me in the direction of an
authenticator that will be able to authenticate against a file in that
format? Alternatively I can alter the format of the file, but the
username and password come from the first server as they are and I can't
unencode them to change them to a different style.

Thanks,
Colin.