zoho blocks a few of my emails if sent using pop but same work if I use zoho's  web based email interface. What gives?   These are personal emails to real world business, no spam or mass mailings.

More specifically, I am able to use thunderbird client / POP for almost all email sends but I get the following rejection on some.  If I send the very send email via web, it works. 

An error occurred while sending mail. The mail server responded:  5.1.8 Email Outgoing Blocked. <a href= https://www.zoho.com/mail/help/usage-policy.html target=_blank>Learn more.</a>. Please check the message and try again.