I keep getting this email for several times a day when I'm trying to send email to my clients. Could you please tell me what's the problem I'm dealing with here, the email doesn't get sent or anything?

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 Sat, 27 Feb 2016 22:05:24 -0800 
from  sale@usatechshop.com  [ sale@usatechshop.com ] 

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

Message will be retried for 4 more day(s) 

 

Thanks