I didn't see this when it was originally posted, but for anyone who looks for this later...
1) Create a mail rule to journal "all documents". (If you want to respond only to internet mail, and you have a separate internet mail server, put it there. Otherwise, add a condition that the domain is not your domain.) You will want this rule to appear AFTER any rules that filter out spam, etc.
2) Put an agent in the mail journal database, set for "before new mail arrives", with the Simple Action set "reply to sender". Put your "business is closed for the holiday" message in the reply. You have the option of "reply only once per person"...I recommend that.
Enable at the start of a holiday, and disable after.
Keep in mind that this WILL reply to spam, which often comes from non-existent email addresses. So, you're going to get a lot of dead mail backed up in your mail.box upon your return. This doesn't happen as much with OOA because it has to be deliverable to your user before it replies.
Goblal Out of office (Budi Febrianto 13.Apr.09)
Feedback response number WEBB7TRHYU created by Maria Helm on 07/08/2009
. . rule, journal, and agent (Maria Helm 8.Jul.09)