ResponseCode 421, Host not reachable

ResponseCode 421, Host not reachable

Getting below message since yesterday, why?

The original message was received at Sun, 30 Oct 2016 09:14:25 -0700  from nnn@comox.net [nnn@comox.net]

    ----- The following addresses had fatal errors -----   

   [Status: Error, Address: <nnnnnnn@shaw.ca>, ResponseCode 421, Host not reachable.]

Message will be retried for 4 more day(s)

    ----- Transcript of session follows -----