Reason:554 5.1.8 Email Outgoing Blocked Unable to send mail

Reason:554 5.1.8 Email Outgoing Blocked Unable to send mail

My colleague is not able to access his email id akshay@shakedeal.com

Reason:554 5.1.8 Email Outgoing Blocked

His inbox is only 1500MB. Need you to unblock it ASAP