I've browsed this forum endlessly and read countless posts and I cannot figure how to fix the problem. My postfix is setup step by step based on the ISPConfig setup for Hardy Ubuntu. The issue is that when I try to send emails to the users in postfix i get a Delivery Status Notification (Failure). It states: When I send a email from the postfix module on the server, I am able to successfully send out emails. Except the email that gets sent out is from the user firstname.lastname@example.org. And when I try to reply to that email I do not immediately get a Delivery Status Notification..... Hours later I received this: This is my main.cf: Also I cannot log in as any of the users via squirrelmail or roundcube. Anyone have any suggestions? Thanks in advance.