ExchangeOAuth2.md

Using OAuth2 With Exchange (IMAP, POP3 or SMTP)

Quick Index

• Register Your Application with Microsoft
• Authenticating with OAuth2
• Additional Resources

Register Your Application with Microsoft

Go to Microsoft's Quickstart guide for registering an application with the Microsoft identity platform and follow the instructions.

Authenticating with OAuth2

Now that you have the Client ID and Tenant ID strings, you'll need to plug those values into your application.

The following sample code uses the Microsoft.Identity.Client nuget package for obtaining the access token which will be needed by MailKit to pass on to the Exchange server.

var options = new PublicClientApplicationOptions {
    ClientId = "Application (client) ID",
    TenantId = "Directory (tenant) ID",
    RedirectUri = "https://login.microsoftonline.com/common/oauth2/nativeclient"
};
 
var publicClientApplication = PublicClientApplicationBuilder
    .CreateWithApplicationOptions (options)
    .Build ();
 
var scopes = new string[] {
    "email",
    "offline_access",
    "https://outlook.office.com/IMAP.AccessAsUser.All", // Only needed for IMAP
    //"https://outlook.office.com/POP.AccessAsUser.All",  // Only needed for POP
    //"https://outlook.office.com/SMTP.Send", // Only needed for SMTP
};

var authToken = await publicClientApplication.AcquireTokenInteractive (scopes).ExecuteAsync ();

var oauth2 = new SaslMechanismOAuth2 (authToken.Account.Username, authToken.AccessToken);

using (var client = new ImapClient ()) {
	await client.ConnectAsync ("outlook.office365.com", 993, SecureSocketOptions.SslOnConnect);
	await client.AuthenticateAsync (oauth2);
	await client.DisconnectAsync (true);
}

Note: Once you've acquired an auth token using the interactive method above, you can avoid prompting the user if you cache the authToken.Account information and then silently reacquire auth tokens in the future using the following code:

var authToken = await publicClientApplication.AcquireTokenSilent(scopes, account).ExecuteAsync(cancellationToken);

Additional Resources

For more inforrmation, check out the Microsoft.Identity.Client documentation.