Cant reply to tickets again

Cant reply to tickets again

for the 5th time in two weeks we cant reply to tickets, We get: 

Error while processing the request! Unable to connect SMTP server:smtpout.mail.zoho.com Reason:Authentication failed:554 5.1.8 Sender Address Blocked
Whatever we tried to send gets saved in draft mode so the ticket isn't updated. We are missing our SLA 'sand customers are getting upset. 
This along with your service outage last week is eroding out faith in the zoho systems. 

Dave