ResponseCode 421, Host not reachable

ResponseCode 421, Host not reachable

Few of my users having this issue, (recipients different domains)
some of error messages,


1/
This message was created automatically by mail delivery system. 

THIS IS A WARNING MESSAGE ONLY. 

YOU DO NOT NEED TO RESEND YOUR MESSAGE. 

----- The following addresses had fatal errors ----- 
[Status: Error, Address: <xxxxxxxxx @utntrade.com >, ResponseCode 421, Host not reachable.] 

Message will be retried for 4 more day(s) 


2/
[Status: Error, Address: <XXXXXXX@slim.lk>, ResponseCode 421, Host not reachable.]