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

First you need to 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 clientSecret = "Client Secret";

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

IPublicClientApplication app = PublicClientApplicationBuilder.Create(clientId)
    .WithAuthority(AzureCloudInstance.AzurePublic, 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 the access token and user email address:

var accounts = await app.GetAccountsAsync();

AuthenticationResult result;
if (accounts.Any())
{
    IAccount account = accounts.FirstOrDefault();
    result = await app.AcquireTokenSilent(scopes, account).ExecuteAsync();
}
else
{
    SecureString securePassword = new SecureString();
    foreach (char c in userPassword)
    {
        securePassword.AppendChar(c);
    }
    result = await app.AcquireTokenByUsernamePassword(
        scopes, 
        userEmail, 
        securePassword).ExecuteAsync();
}

string user = result.Account.Username;
string accessToken = result.AccessToken;

Finally you can connect to IMAP/POP3/SMTP server and authenticate:

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

    // ...

    client.Close();
}