I used a template to create a custom Microsoft Office Outlook rule for out-of-office replies. However, even though the rule fires, the custom out-of-office message doesn't reach everyone who sends me mail while I'm out. Why not?
You've probably configured Microsoft Exchange Server to block automatic replies to the Internet. You can selectively allow or block out-of-office messages, automatic replies to the Internet, automatic forwards to the Internet, and both delivery and nondelivery reports via the settings on the Advanced tab of the properties page for a specific Internet message format (including the default format, which applies to all messages your servers send unless you override it). However, if you disable automatic replies to the Internet, then custom out-of-office rules won't be able to send messages to Internet senders. You'll need to either allow automatic replies to the Internet or use the standard out-of-office message.
—Paul Robichaux