When you try to validate an outbound connector in the Exchange admin center in Office 365, the test email message isn't delivered, and you receive the following error message:
You receive this message even though the domain was added to the accepted domains list.
The domain of the recipient is not configured as part of connector.
The outbound connector was saved without having the option enabled to turn on the connector after it's saved.
On the New Connector page, select the Turn it on check box to turn on the connector after it's saved, save the connector, and then validate it.
Article ID: 3179588 - Last Review: Dec 28, 2016 - Revision: 1
Microsoft Exchange Online, Microsoft Exchange Online Protection