Modify subject based on recipient

[Daniel L. Miller]

Does anyone know of a tool that will let me modify the subject line of 
all emails that pass through it?  I would call it via a transport map.

My application - we just switched to a new email-to-fax service.  As 
part of their security implementation (THEIRS, not mine!) they require 
all emails sent to them to contain our own fax number.  I want to 
automate this step so I don't have to hear from my users.

--
Daniel