ResponseCode 421, Host not reachable

ResponseCode 421, Host not reachable

LS, 

I am getting the error  ResponseCode 421, Host not reachable. That is odd, since on other domains, I can mail these people without a problem. 

I have MX records set correctly, and only zoho MX records, I see not issue with the set-up. can you help? 
regards
Edwin