"Http server returned Forbidden exception" error appears in Dynamics 365 mailbox

When you click the Test & Enable Mailbox button on a mailbox record in Dynamics 365, the test results section shows Failure and the following alert is logged:

"The email message "Your mailbox is now connected to Dynamics 365" cannot be sent because an error occurred while establishing a secure connection to the email server. Mailbox [Mailbox Name] didn't synchronize. The owner of the email server profile Microsoft Exchange Online has been notified.

Email Server Error Code: Http server returned Forbidden exception." 

If you click Details, the following additional details are shown:

"Error : System.Net.WebException: The request failed with HTTP status 403: Forbidden.
   at System.Web.Services.Protocols.SoapHttpClientProtocol.ReadResponse(SoapClientMessage message, WebResponse response, Stream responseStream, Boolean asyncCall)
   at System.Web.Services.Protocols.SoapHttpClientProtocol.EndInvoke(IAsyncResult asyncResult)
   at Microsoft.Crm.Asynchronous.EmailConnector.ExchangeServiceBinding.EndCreateItem(IAsyncResult asyncResult)
   at Microsoft.Crm.Asynchronous.EmailConnector.ExchangeOutgoingEmailProvider.EndCreateItem()"

Dynamics 365 communicates with Microsoft Exchange using Exchange Web Services (EWS) requests. If EWS is disabled, this error will occur. The following are some potential ways EWS may be disabled or restricted in Exchange:

1. EWS is disabled for the mailbox
2. EWS is disabled for the entire organization
3. The EwsApplicationAccessPolicy is set to EnforceAllowList and the EwsAllowList does not allow access from Dynamics 365 (CRM).
4. The EwsApplicationAccessPolicy is set to EnforceBlockList and the EwsBlockList includes Dynamics 365 (CRM).

If the issue only occurs for some mailboxes, check if EWS is disabled for the mailbox:

1. First check to see if EWS has been disabled for the mailbox. Use the following PowerShell command:
   
   Get-CASMailbox <mailbox-alias> | ft EwsEnabled
2. If EwsEnabled is set to False, use the following PowerShell command to enable Exchange Web Services (EWS) for the mailbox:
   
   Set-CASMailbox <mailbox-alias> -EwsEnabled $True
   
   IMPORTANT: After running this command, it may take up to 120 minutes before the setting change takes effect. 

If the issue occurs for all mailboxes, check if EWS is disabled at the organization level or if the EwsAllowList is being used to limit what EWS traffic is allowed.

1. Use the following PowerShell command to see if any of the EWS settings are configured:
   
   Get-OrganizationConfig |ft Name, EwsEnabled,EwsApplicationAccessPolicy,EwsBlockList,EwsAllowList
    
2. Verify that EwsEnabled is not set to False. The following command can be used to set EwsEnabled to True if it is currently set to False:
   
   Set-OrganizationConfig -EwsEnabled $True
   
   IMPORTANT: After running this command, it may take up to 120 minutes before the setting change takes effect. 
    
3. If EwsApplicationAccessPolicy is set to EnforceAllowList and the EwsAllowList does not contain a value for CRM (Example: CRM/*), this would prevent Dynamics 365 (CRM) from being able to communicate with Exchange. Use the following command to update the list to include CRM/* and whatever other applications you want to allow (<PreviousAllowList> in the following example):
   
   Set-OrganizationConfig -EwsApplicationAccessPolicy:EnforceAllowList -EwsAllowList:CRM/*,<PreviousAllowedList>
   
   IMPORTANT: After running this command, it may take up to 120 minutes before the setting change takes effect. 
    
4. If EwsApplicationAccessPolicy is set to EnforceBlockList and the EwsAllowList contains a value for CRM (Example: CRM/*), this would prevent Dynamics 365 (CRM) from being able to communicate with Exchange. Use the following command to update the list to no longer include CRM:
   
   Set-OrganizationConfig -EwsApplicationAccessPolicy:EnforceBlockList -EwsBlockList:<PreviousBlockList WITH CRM REMOVED>
   
   IMPORTANT: After running this command, it may take up to 120 minutes before the setting change takes effect. 

See the following articles for additional information about changing Exchange settings using PowerShell and controlling access to EWS:

Exchange Server PowerShell (Exchange Management Shell)
https://docs.microsoft.com/powershell/exchange/exchange-server/exchange-management-shell?view=exchange-ps 

Connect to Exchange Online PowerShell
https://docs.microsoft.com/powershell/exchange/exchange-online/connect-to-exchange-online-powershell/connect-to-exchange-online-powershell?view=exchange-ps

Control access to EWS in Exchange
https://docs.microsoft.com/exchange/client-developer/exchange-web-services/how-to-control-access-to-ews-in-exchange 

Set-CASMailbox
https://docs.microsoft.com/powershell/module/exchange/client-access/set-casmailbox?view=exchange-ps