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OAuth2 Authentication to send emails (C# or VB.NET)


My company is looking for a .NET component that will send emails using OAuth2 authentication (we code in C# and VB.NET).
Until now we used SMTP with basic authentication and as we know Microsoft will very soon discontinue this method of authentication.
So now we need to use OAuth2. We do not have the choice.

In order to be able to send emails from our home-made developments, our O365 administrator registered an application with Azure and sent us the following information:

  • Secret Value
  • Secret id
  • App id
  • tenant ID

Looking through your site, the ‘Aspose Email’ component seems to meet our needs.
We have downloaded the trial version but we are unable to use the code samples available on your site.
This new authentication method is quite complicated.
To be honest, we are tearing our hair out :frowning:

Is there a complete and detailed code that allows to send an email (Office 365 mailbox) with OAuth2? Also, our home-made application that sends email is a service. So the application has to be able to authenticate without a signed-in user present.

If the component meets our expectations, we will surely place an order.

Thank you for your precious help.


Please read the following detail to achieve your requirement.

How To Enable or disable modern authentication

To use modern authentication, make sure that it is enabled. Modern authentication is enabled by default in Exchange Online. For tenants created before August 1, 2017, modern authentication is turned off by default.
In the Microsoft 365 admin center at https://admin.microsoft.com, go Settings > Org Settings > Modern Authentication. In the Modern authentication flyout that appears, you can identify the protocols that no longer require Basic authentication.
For new Office 365 tenants in Azure, Basic Authentication is disabled by default for all applications. In this case, the text will be displayed in this section.

Your organization has security defaults enabled, which means modern authentication to Exchange Online is required, and basic authentication connections are blocked. You must turn off security defaults in the Azure portal before you can change any settings here.

You can enable Basic Auth support for tenant from the Azure portal (Azure Active Directory -> Properties -> Manage Security defaults -> Enable Security defaults = No ).
For more information, see the documentation on https://docs.microsoft.com/en-us/exchange/clients-and-mobile-in-exchange-online/enable-or-disable-modern-authentication-in-exchange-online

How To use modern authentication with EwsClient

To use modern authentication with EwsClient the following is required:

  1. App registration with Azure Active Directory.
  2. Adding code to get an authentication token from a token server.
  3. Using the token to authenticate.

Note: There are two types of permissions that can be used to access EWS. Choose a specific type of permission, depending on the app you are creating:

  • Delegated permissions are used by apps that have a signed-in user present. For these apps, either the user or an administrator consents to the permissions that the app requests. In other words, when you connect to the service, a dialog window will appear to enter username and password. App can never have more privileges than the signed-in user.
  • Application permissions are used by apps that run without a signed-in user present, for example, apps that run as background services or daemons. Only an administrator can consent to application permissions.

Refer to Microsoft documentation for more information: https://docs.microsoft.com/en-us/exchange/client-developer/exchange-web-services/how-to-authenticate-an-ews-application-by-using-oauth

App registration with Azure Active Directory

The registration procedure depends on the type of permission selected. To register your app, refer to the Microsoft documentation:

You can download full code example to use modern authentication with IMAP and SMTP clients from here:
EWSModernAuthenticationImapSmtp.zip (3.9 KB)

You can also download full code examples to use different permission types of modern authentication with EWS client from here:

With Delegated authentication - EWSModernAuthenticationDelegated.zip (3.6 KB)
With Application authentication - EWSModernAuthenticationApp.zip (3.4 KB)