Zoho rejected email sent from SendInBlue with 554 5.2.3 MailPolicy violation Error delivering to mailboxes

Hi,

I am using Zoho email server for my SendInBlue account, to send transnational emails for my web app. Yesterday, 09-04-2020 19:47:43 my email sent by SendInBlue was rejected with 554 5.2.3 MailPolicy violation Error delivering to mailboxes. Email size was around 50KB, so I do not see why this would happen. Can you tell me what seems to be the problem and how I can fix this.

Thanks

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