OAuth 2.0 with Office365/Exchange IMAP/POP3/SMTP

This article shows how to implement OAuth 2.0 desktop flow to access Office365 via IMAP, POP3 or SMTP using Mail.dll .net email client.

Make sure IMAP/POP3/SMTP is enabled for your organization and mailbox:
Enable IMAP/POP3/SMTP in Office 365

Register your application in Azure Portal, here’s a detailed guide how to do that:
https://docs.microsoft.com/en-us/azure/active-directory/develop/quickstart-register-app

Remember to add authentication entries (localhost is needed for .net core):

RedirectUri

.NET desktop: https://login.microsoftonline.com/common/oauth2/nativeclient
.NET core/.NET 5,6,7+: http://localhost
ASP.NET: your application custom url

Then you need to apply correct API permissions and grant the admin consent for your domain.

In the API permissions / Add a permission wizard, select Microsoft Graph and then Delegated permissions to find the following permission scopes listed:

  • offline_access
  • email
  • IMAP.AccessAsUser.All
  • POP.AccessAsUser.All
  • SMTP.Send

Remember to Grant admin consent..

Use Microsoft Authentication Library for .NET (MSAL.NET) nuget package to obtain an access token:
https://www.nuget.org/packages/Microsoft.Identity.Client/

string clientId = "Application (client) ID";
string tenantId = "Directory (tenant) ID";

// for @outlook.com/@hotmail accounts instead of setting .WithTenantId use:
// .WithAuthority(AadAuthorityAudience.PersonalMicrosoftAccount)

var app = PublicClientApplicationBuilder
                .Create(clientId)
                .WithTenantId(tenantId)
                .WithDefaultRedirectUri()
                .Build();
// This allows saving access/refresh tokens to some storage
TokenCacheHelper.EnableSerialization(app.UserTokenCache);

var scopes = new string[] 
{
    "offline_access",
    "email",
    "https://outlook.office.com/IMAP.AccessAsUser.All",
    "https://outlook.office.com/POP.AccessAsUser.All",
    "https://outlook.office.com/SMTP.Send",
};

In addition, you should request offline_access scope. When a user approves the offline_access scope, your app can receive refresh tokens from the Microsoft identity platform token endpoint. Refresh tokens are long-lived. Your app can get new access tokens as older ones expire.

Now acquire the access token and user email address:

string userName;
string accessToken;

var account = (await app.GetAccountsAsync()).FirstOrDefault();

try
{
    AuthenticationResult refresh = await app
        .AcquireTokenSilent(scopes, account)
        .ExecuteAsync();

    userName = refresh.Account.Username;
    accessToken = refresh.AccessToken;
}
catch (MsalUiRequiredException e)
{
    var result = await app.AcquireTokenInteractive(scopes)
        .ExecuteAsync();

    userName = result.Account.Username;
    accessToken = result.AccessToken;
}

Finally you can connect using IMAP/POP3/SMTP, authenticate and download user’s emails:

using (Imap client = new Imap())
{
    client.ConnectSSL("outlook.office365.com");
    client.LoginOAUTH2(userName, accessToken);
 
    client.SelectInbox();

    List<long> uids = imap.Search(Flag.Unseen);
    foreach (long uid in uids)
    {
        IMail email = new MailBuilder()
                .CreateFromEml(imap.GetMessageByUID(uid));
        string subject = email.Subject;
   }

    client.Close();
} 

As this is fairly new feature for Exchange/Office365, here are some useful links:

https://docs.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth

https://stackoverflow.com/questions/29747477/imap-auth-in-office-365-using-oauth2

https://stackoverflow.com/questions/43473858/connect-to-outlook-office-365-imap-using-oauth2

https://stackoverflow.com/questions/61597263/office-365-xoauth2-for-imap-and-smtp-authentication-fails

Token serialization

Below is a simple implementation that saves MSAL token cache to file:

static class TokenCacheHelper
{
    public static void EnableSerialization(ITokenCache tokenCache)
    {
        tokenCache.SetBeforeAccess(BeforeAccessNotification);
        tokenCache.SetAfterAccess(AfterAccessNotification);
    }

    private static readonly string _fileName = "msalcache.bin3";

    private static readonly object _fileLock = new object();


    private static void BeforeAccessNotification(TokenCacheNotificationArgs args)
    {
        lock (_fileLock)
        {
            byte[] data = null;
            if (File.Exists(_fileName))
                data = File.ReadAllBytes(_fileName);
            args.TokenCache.DeserializeMsalV3(data);
        }
    }

    private static void AfterAccessNotification(TokenCacheNotificationArgs args)
    {
        if (args.HasStateChanged)
        {
            lock (_fileLock)
            {
                byte[] data = args.TokenCache.SerializeMsalV3();
                File.WriteAllBytes(_fileName, data);
            }
        }
    }
};

Please note that most likely you should store this cache in an encrypted form in some kind of a database.
Consider using MSAL token serialization implementations available here:

https://docs.microsoft.com/en-us/azure/active-directory/develop/msal-net-token-cache-serialization

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