Some certain gmail are having this error .Please help

Some certain gmail are having this error .Please help

This message was created automatically by mail delivery system. 

THIS IS A WARNING MESSAGE ONLY. 

YOU DO NOT NEED TO RESEND YOUR MESSAGE. 

The original message was received at Mon, 23 Nov 2015 22:57:19 -0800 
from  greg@xx.com  [ greg@xxx.com ] 

----- The following addresses had fatal errors ----- 
[Status: Error, Address: < xxxx@gmail.com >, ResponseCode 421, 4.7.0 Temporarily Deferred]