It looks like the recipient isn't a valid email address in your organization's address book. Check the email address spelling ...

It looks like the recipient isn't a valid email address in your organization's address book. Check the email address spelling for the Recipient: {Recipient}, and run the checks again.
If you entered the intended recipient address, they're not receiving email because they don't exist in your email system. To fix this, you need to add them to Office 365 using the following steps.
  1. In the Office 365 Admin center, go to the Active users page.
  2. Choose Add a user.
  3. Provide the requested information, and a mailbox is created automatically for the new user.