By using the trick of plus addressing a single Gmail account can take care of correct and incorrect answers. Each incoming mail will be rewarded by a feedback mail.

The standard auto-response / vacation responder was not sufficient. To send two different responses, I used the Mail Labs feature "Canned Responses"...

...and saved two different response drafts.

An email addressed to the correct recipient (containing the proper date after the plus) will be labeled with "richtig" (German for "correct"). The sender gets the matching canned response and the mail gets forwarded to my address. So I get notified when someone is in front of our building and is looking for the cache ;)