OAuth 2.0 password grant with Office365/Exchange IMAP/POP3/SMTP

This article shows how to implement OAuth 2.0 password grant 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

Password grant flow requires Mulit-Factor Authentication (MFA) to be disabled for this mailbox – make also sure there are no Active Directory policies that match this account and require MFA (you can of course have policies that match all other accounts).

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

Enable additional flows:

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";

string userEmail = "Username for mailbox";
string userPassword = "Password for that user";

IPublicClientApplication app = PublicClientApplicationBuilder
    .Create(clientId)
    .WithTenantId(tenantId)
    .Build();

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",
};

Now acquire an access token and a user name:

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)
{
    SecureString securePassword = new SecureString();
    foreach (char c in userPassword)
    {
        securePassword.AppendChar(c);
    }

    var result = await app.AcquireTokenByUsernamePassword(
        scopes, 
        userEmail, 
        securePassword).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();
} 

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