"Sending replies automatically
You can create a rule that makes the Mail application automatically reply to incoming messages. You can have Mail add new text to the reply before it is sent, such as an explanation that the message was automatically generated.
The automatic reply feature may be useful if you go on vacation or can't check your email for a long period of time. You can have Mail automatically reply to any mail you receive with a message explaining where you are or when you'll return.
Choose Mail > Preferences and 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 is sent.
After a rule has processed a message, the message's status icon will change to a gray arrow, showing that a reply was sent.
Carefully consider the rules you specify 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.