sendmail (a mail transport agent - MTA) stores mail in the place where it is configured to store mail. But an MTA only stores mail which it is currently queueing - delivery of mail to the location where it is accessible by a user is handled by the mail delivery agent (MDA). While sendmail (and other MTAs) can act as an MDA, it's very unusual for that to be the case.
Regardless, trying to access the mail files is not the right way to implement an autoresponder.
This should be done as a filter in the MTA (i.e. a milter) or in the MDA. The milter API is specifically designed for this - and there may already be tools available which would meet your requirement with no modification.
If your current MDA is procmail then in you're in luck - this provides a huge amount of flexibility (indeed its a programming language in its own right) for handling mail messages. Again there is lots of code on the internet which may meet your needs (AFAIK there is no central respository for procmail recipes - but try google).
Even the basic .forward mechanism can be used to implement this - but I'd advise against this - goingdown the procmail route you can fork the message so that delivery is not dependent upon/implemented by your script.