I'm receiving "Unroutable Address" bounce back error messages

The error "Unroutable address / Unroutable mail domain" is a general error which can be caused by several things:

• A misspelled email address.  
  If you mis-spell the email address of the domain, and the mis-spelled domain does not exist, you will receive this message.  Please check the spelling of the email address of your recipient and try sending the message again
   
• The sending email server cannot determine the correct route for sending the email. 
  This is usually caused by a DNS error with the recipients MX records.  We cannot fix this issue and the recipient will need to be contacted by some other method and informed of the error in their DNS records.