Email is not sent from Dynamics 365 and the following alert is found within the mailbox record:
"An unknown error occurred while sending the email message [Email Subject] through the mailbox [Mailbox Name]. The owner of the associated email server profile Microsoft Exchange Online has been notified. The system will try to send the message again later.
Email Server Error Code: Exchange server returned UnknownOutgoingEmailIntegrationError ErrorSubmissionQuotaExceeded exception."