If you have enabled Constituents settings, then if you enable an autoreply here, when non-constituents (or people who do not provide a parseable address)
send repetitive emails for which you have trained responses intended for your constituents, they will get this standard reply instead of your trained reply.  This autoreply only replies to trained incoming emails.  It is intended 
to filter out non-constituents from incoming mass email campaigns.  It does not take the place of a general purpose auto reply.  New constituents will not be created and no activity record will be saved when an autoreply is issued.