Relaying disallowed | Message distribution : Authorization denied

Relaying disallowed | Message distribution : Authorization denied

Domain name: msparkservices.com
Domain is verified and configured with zoho and has been in use for the last 2 months. 
For the last couple of day we have been encountering the following issues:-
  1. Relaying disallowed
    • This error is encountered when an email is being sent from zohomail (msparkservices.com) to any address outside of our domain.
    • Email sending fails.
    • Emails can be sent to other zoho accounts inside msparkservices.com.
  2. Message distribution : Authorization denied
    • This error is encountered when an email is being sent to zohomail (msparkservices.com) from any email address outside of this domain.
    • Email is bounced with a error message. (PFA -- screenshot of the 'Authorization denied' error message.
    • Emails sent from zoho (msparkservices.com) are getting delivered to other zoho accounts inside the domain (msparkservices.com).
We have verified the status of all server DNS records (MX, CNAME etc.) and everything seems to be in order.
Please suggest a way to resolve this issue.