autoreply

[Herr Vinny]

Has anyone developed a mailet to autoreply to messages? I'll still be rolling my own 
mailet (my customer need some unique functionality) but I wouldn't mind some pointers. 
Also, what happens when James receives an email that it can't deliver? Doesn't it send 
out an "unable to deliver" message? and if it does, where is it defined? Thanks.