Read the errors:
host name lookup failure - The MX record for the destinations domain probably contains a dummy/silly host
Sender address rejected - The receiving mail server doesn't like you.
Domain not found - Trying to send to a non-existant domain.
Deferred - Something else happened, this mail will be resent at some point
Closing connection - Remote end closed the connection without accepting the mail
no valid recipients - Remote end can't find the person you're sending to
No route to host - Your system can't make a connection to the remote end, may be a network issue or something.
etc..... - etc.
Postfix will retry delivery and eventually give up and delete the mail from the queue.
It would be normal for a busy mail server to have a number of these mails sitting in the queue.
Last edited by TenTenths; 01-17-2014 at 06:37 AM.