[Exim] Serialmail and Exim

I'm attempting to support dial-in customers on an Exim platform. I have got
Exim to store such messages in maildir format. When a customer connects, I
use Serialmail to scan their maildir and send each message to their host
through SMTP. All works well except for handling delivery failures.
Serialmail uses qmail to send bounce messages back to the sender, but
naturally I don't want to install qmail on our Exim servers.

Has anyone modified Serialmail to send messages through Exim? The author of
Serialmail was not overly generous with comments in his code!!

Thanks in advance

Brian