Domain Not resolved

Domain Not resolved

Dear Sir,

I have a urgent problem when sending email to one of our user (me). 
We received emails like this : 

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. 


81300000000@efaxsend.com Domain Not Resolved
Can you help us asap?

Thanks