When using Gmail to send email from Dynamics 365, emails are not sent and appear in a Pending Send status. If you open the mailbox record in Dynamics 365 and click Alerts, you see the following warning:
"An unexpected error occurred while sending the email message [Email Subject] through the mailbox [Mailbox Name]. The owner of the associated email server profile [Email Server Profile name] has been notified. The system will try to send the message again later.
Email Server Error Code: Smtp server returned GeneralOutgoingEmailServerError GeneralFailure exception."