Sendmail, Procmail, and Majordomo

Hi, 

I am pretty new to all this but I have a question.  I have a working
majordomo listserver and Procmail and I am trying to automatically Prepend a
string of text to the outgoing messages from the mailing list.  I have
figured out basically how to use proc mail from a user's account point of
view but how would I invoke it to process outgoing listserver mail?

Thanks in advance,

Hedley Robertson
US Army, Computer Science School