Mail being blocked for Throttling?

Mail being blocked for Throttling?

I sent an email to a client domain that I have sent email in the past. Today I was replying to a client's support ticket through our contact form and when I sent said client a reply I received this. (client info redacted)

 The following addresses had fatal errors ----- 
[Status: THROTTLED, Address: < xxxxxxxx@xxxxxxxxx.com>, ResponseCode 421, 4.7.0 Temporarily Deferred] 

I certainly not sending enough email to trigger a throttling alert. Furthermore I sent a test to my personal hotmail and email accounts and the email was completely rejected from Gmail. In other words, it didn't even land in the junk folder. 

What is going on Zoho? Is it possible that you guys can go 10 days without a major issue with your email offering?