Microsoft Outlook 2011 for MAC, could not send email? I am getting "Mail Delivery Failed" messages for some of the
emails sent.

As I am replying an email, I am sure that there isn't any misspelling on the recipient's email address.

A typical message below.  

On 22/06/2015 9:47 am, "Mail Delivery System"
 wrote:


  This message was created automatically by mail delivery software.
  
  A message that you sent could not be delivered to one or more of its
  recipients. This is a permanent error. The following address(es) failed:
  ...


Any help would be very much appreciate.

Thanks in advance.
It is possible that the mail was sent from a disposable e-mail address. If that's the case then mail sending will be failed.