Argh ... anyone here knows Exchange 5.5 better than I do [which shouldn't be too hard]?

I'm preparing to leave for Barcelona (TechEd) and some weeks of vacation (Norway) and wanted to set up these Out-Of-Office autoreplies for the first time. Bad thing is - it only works when the incoming email has been sent from another user of our Exchange server but not when I receive an email from the internet. 

Anyone has any hints or ideas? Thanks!