If you use the Out Of Office Assistant and turn on the out-of-office reply, the internal senders (on the local network) receive the automatic reply message, but external senders (on the Internet) may not receive the automatic reply.
By default, the out-of-office reply to Internet recipients is turned off.
To turn on out-of-office replies to the Internet, follow these steps:
On the Exchange server, start Exchange System Manager.
Double-click Global Settings, and then click Internet Message Formats.
In the Details pane, right-click the Domain name, and then click Properties. The default SMTP domain is “*”.
Then open up Services in Administrative tools and Restart the Simple Mail Transfer Protocol (SMTP) and Microsoft Exchange Routing Engine services.