After you click the Test & Enable Mailbox button in Dynamics 365, the tests fails and the following alert is logged:
"The email message "Your mailbox is now connected to Dynamics 365" cannot be sent because the mailbox [Mailbox Name] is deactivated or disabled for outgoing email.
Email Server Error Code: MailboxDisabled"
This error occurs if the mailbox is deactivated and the status is now Inactive.
A mailbox could have been deactivated by someone clicking the Deactivate button. If this is a user mailbox, the mailbox could have been deactivated automatically if the regarding user was deactivated. For example: If you remove a user's Dynamics 365 license, the user will be deactivated and also their mailbox record.
- Click the Activate button on the mailbox record and verify the status shown in the footer of the page shows as Active.
- Click the Test & Enable Mailbox button.
- If the tests do not succeed, review the alerts that appear.