I was trying to email a fellow youtuber and it stopped allowing me to send him messages :(

I was trying to email a fellow youtuber and it stopped allowing me to send him messages :(

I am trying to collaborate with another youtuber so we were emailing each other. All of a sudden I sent him 2 songs asking if he liked either and i got this message:

Technical details of permanent failure:
Google tried to deliver your message, but it was rejected by the server for the recipient domain zoho.com by smtpin.zoho.com. [165.254.168.100].

The error that the other server returned was:
554 5.2.3 MailPolicy violation Error delivering to mailboxes
Why did this happen and if you know please help me fix it