Unable to Send Emails since 11.01.2017

Unable to Send Emails since 11.01.2017

Hello Zoho,

We are unable to send emails since 11.01.2017, first we thought its a temporary issue but now it seems it isn't.

While sending an email we are receiving an error " Unable to send message;Reason:554 5.1.8 Sender Address Blocked"
Screen-shot attached
Please resolve this issue ASAP, we are loosing business because of your horrible services.
And please make sure these issues never occur in future.

Thanks and Regards,
Manfred Infotech