CRM user when click on the primary email address of any record from CRM then email is generated, it opens in the OWA window.
Customer click on the Dynamics 365 tab (which is for CRM App for outlook) from email, then the contact is not detected and considered as a new email, it gives option of adding it to Dynamics 365 and it shows recipient as Unknown Recipient however when we create a new email directly from OWA and click on Dynamics365, then it shows CRM record and all associated records of CRM org.
Also same could be seen when you send email from the OWA or outlook and then go to Sent item and open the sent email.
Here when click on the Dynamics 365 tab (which is for CRM App for outlook) it doesn't resolve the recipient and gives the message "Unknown recipient" .
when we click on Dynamics 365 tab (which is for CRM App for outlook) then it should automatically resolve the recipient as the Outlook is already associated with CRM using CRM App for Outlook