Email sent from one cpanel email account to another user is rejected with the error message:
550 5.7.133 RESOLVER.RST.SenderNotAuthenticatedForGroup;
authentication required; Delivery restriction check failed because the sender was not authenticated when sending to this group (in reply to RCPT TO command)
This error message is self-explanatory and indicates that the recipient address is configured to only allow authenticated senders. Because your domain is not authenticated to the recipient’s domain, the email is blocked.
This is a usual occurrence with email lists (distribution groups), for example when you try to send an email to a list that you are not part of, your email will either be held in the queue for approval by the List admin or rejected.