The following warning level alert appears within a Dynamics 365 mailbox record:
"An error occurred in sending email for mailbox [Mailbox Name] while connecting to the email server. The owner of the associated email server profile [Profile Name] has been notified. The system will try to send email again later.
Email Server Error Code: Web server returned SendFailure exception."
This warning may appear if Server-Side Synchronization attempted to send an email but encountered an error while connecting to your email server. This is typically a temporary issue and the item will be sent successfully when the item is retried.
If this message does not continue to occur, it can be ignored. If this message continues to be logged and you have emails that won't send successfully, you can contact Microsoft Support for assistance. There may be an issue with your email server so you can first verify you can send and receive email successfully with your email service.