To automatically reply to messages:
Choose Mail > Preferences, and then click Rules.
Click Add Rule and type a descriptive name for the rule.
Set the conditions necessary for the rule to apply to a message.
Choose “Reply to Message” from the “Perform the following actions” pop-up menu.
Click “Reply message text” and enter the text for your reply message. Mail will include the text you enter, along with the full text of the original message, in the message that it sends.
Mail must be running to automatically reply to messages, because rules aren’t processed while your computer is shut down or you’re logged out. To set up automatic replies while you’re on vacation, ask your Mail server’s administrator how to set up an out-of-office reply.
***Carefully consider the rules you create for sending automated replies. If they’re too broad, you can send messages inadvertently or create message loops (replies to replies you sent). If possible, test the rule.