Hi Gavin ,
iam sending a few links for ur reference. Please check them out and see if they suit ur demands.
look procmail will help u in autoresponding to a particular account or on any other criteria that matches however ur MTA ( sendmail ) will by default reject unknown users so there is no way procmail will come into picture ( so no autoresponder !!!!!! )
so the first thing for u to do is setup ur mail server so that it will accept all email ( even if user does not exsist) and forward it to some local account on ur machine and then procmail comes into picture and does autoresponding part.
u can accomplish the allowing of mails to non local users by making changes in the /etc/aliases file or thru sendmail's various *table features.
Hope this eases a bit for u.