Linux Mail Server Setup and Howto Guide

Hi,

i followed the article "postfix Vacation Autoresponder" to configure auto respond messages in my server but when i try to send an email to test@vacation.invalid i get Undelivered Mail Returned to Sender :

<test@vacation.invalid>: Host or domain name not found. Name service error for
    name=vacation.invalid type=A: Host not found

and when i try to send an email to an actual user who has an auto respond message it says the same message but the email get delivered.

any help will be appreciated.

thanks.