I'm using the free version of Zoho Mail with my custom domain. I haven't enforced any email policy via the admin control panel. However, I'm getting the following sender address block error.

SMTP -> get_lines(): $str is  "554 5.1.8 Sender Address Blocked
"
SERVER -> CLIENT: 554 5.1.8 Sender Address Blocked

SMTP ERROR: Password command failed: 554 5.1.8 Sender Address Blocked

SMTP Error: Could not authenticate.
CLIENT -> SERVER: QUIT

I'm using the email address with an internal project management tool and it does send frequent notification emails to project members. 

Can any official support person clarify if there is any limit enforced? If not then why I do encounter this sender blocked issue every now and then?