When attempting to send an email in Dynamics 365, the email remains in a Pending Send status and the following alert is logged in the user's mailbox record:
"The email message [Message Subject] cannot be sent. Make sure that the credentials specified in the mailbox [Mailbox Name] are correct and have sufficient permissions for sending email. Then, enable the mailbox for email processing.
Email Server Error Code: ErrorQuotaExceeded. Exchange.server returned 403 error."