[ASP.NET CORE] Exchange Web Services Throw HTTP 401 When Called from IIS

Hi,

Currently, I can't get Microsoft.Exchange.WebServices to work with Exchange 2019 On-Premise. Send Email feature is working OK on Development but as soon I deployed it to IIS, it stopped working with following error:

Microsoft.Exchange.WebServices.Data.ServiceRequestException: The request failed. The remote server returned an error: (401) Unauthorized.
---> System.Net.WebException: The remote server returned an error: (401) Unauthorized.
   at System.Net.HttpWebRequest.GetResponse()
   at Microsoft.Exchange.WebServices.Data.EwsHttpWebRequest.Microsoft.Exchange.WebServices.Data.IEwsHttpWebRequest.GetResponse()
   at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.GetEwsHttpWebResponse(IEwsHttpWebRequest request)
   --- End of inner exception stack trace ---
   at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.GetEwsHttpWebResponse(IEwsHttpWebRequest request)
  at Microsoft.Exchange.WebServices.Data.ServiceRequestBase.ValidateAndEmitRequest(IEwsHttpWebRequest& request)
   at Microsoft.Exchange.WebServices.Data.SimpleServiceRequestBase.InternalExecute()
   at Microsoft.Exchange.WebServices.Data.MultiResponseServiceRequest`1.Execute()
   at Microsoft.Exchange.WebServices.Data.ExchangeService.InternalCreateItems(IEnumerable`1 items, FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode, ServiceErrorHandling errorHandling)
   at Microsoft.Exchange.WebServices.Data.ExchangeService.CreateItem(Item item, FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode)
   at Microsoft.Exchange.WebServices.Data.Item.InternalCreate(FolderId parentFolderId, Nullable`1 messageDisposition, Nullable`1 sendInvitationsMode)
   at Microsoft.Exchange.WebServices.Data.EmailMessage.InternalSend(FolderId parentFolderId, MessageDisposition messageDisposition)

I'm using same Exchange Settings (URL, Credentials, etc.) for both instance but it is only worked on Development. My Site is using App Pool User which is registered on Exchange Mailbox Users. How to fix this issue?

Thanks in advance.

Best Regards,

Henoch