From memory...
Tools | Rules and Alerts
New Rule
Start from a blank rule
Check messages as they arrive | Next button
Select Next and answer yes to the question about having it apply to all
messages
Have server reply using a specific message
Click on the link at the bottom of dialog to create message text (leave
subject blank, but type what you want as a reply)
Next | Finish at this point
Keep in mind that this might not work if testing between Exchange mailbox
and an ISP mail account. The reason why is that there are options on the
Exchange server to turn off auto-reply and auto-forward actions to the
internet.