Test connection fails for Exchange Web Services(EWS) endpoint with Office 365 Sync

Problem

Test connection fails for Exchange Web Services(EWS) endpoint with Office 365 Sync.

Error Message

Test connection success: Office 365 Powershell Credentials. Exchange Web Services connection attempt failed, check your information and try again. Test connection success: Graph API Configuration. 

Cause

The Office 365 Sync checks connections to PowerShell, Exchange Web Services & Graph API endpoints against an anchor mailbox. This anchor mailbox is generally the first mailbox in the directory. To determine which mailbox will be used for the check execute the following PowerShell cmdlet while connected to the O365 tenant. 

Get-MailBox -ResultSize 1 -RecipientTypeDetails UserMailbox | Select-Object DisplayName, PrimarySmtpAddress, UserPrincipalName , WhenCreated | FL

If the anchor mailbox is not licensed the EWS connection will fail. 

Solution

Assign a license to the mailbox returned as a result of running the above query. Once done, test the O365 Sync connection again. 

Note: Preferably assign a M365 E3 or E5 license to this mailbox.