Error 550 5.4.6 unable to send emails

Error 550 5.4.6 unable to send emails

I am using the CC field to send the same email to 15 people, using the web version of Zoho Email. Is this the problem?
I don't want to copy/paste the same email and I don't have a CRM.
Last year we have used this account to send some automatic emails but it's no longer used for it