How to Set Up Auto Reply

Just picked up a copy of procmail-3.10....

I want to use it to send an auto-acknowledgement to all incoming
email messages.  What's the best way to do this?

I've found that if I put the following in my .forward:

 \user, "| formail -r | /usr/lib/sendmail -t"

then I sort of get what I want, except that I can't specify a 
message to send back with.  Though formail is mentioned as the
way to do auto-replies, there doesn't seem to be any example on
how this is actually done.

George