Authentication failed exception with MailKit OAuth2.0

I'm using the following code to get an access token and connect to the mail folder:

var confidentialClientApplicationBuilder = ConfidentialClientApplicationBuilder.Create(clientId).WithClientSecret(clientSecret).WithTenantId(tenantId).Build();var scopes = new string[] { ".default" };var authToken = await confidentialClientApplicationBuilder.AcquireTokenForClient(scopes).ExecuteAsync();var oauth2 = new SaslMechanismOAuth2(username, authToken.AccessToken);using (ImapClient client = new ImapClient()){    await client.ConnectAsync("outlook.office365.com", 993, SecureSocketOptions.SslOnConnect);    await client.AuthenticateAsync(oauth2);    //TODO    await client.DisconnectAsync(true);}

Everything seems to work correctly here, the ImapClient is connected and I can see oauth2.Credentials.Password is populated with the access token. However, when I run it the AuthenticateAsync method throws the error:

MailKit.Security.AuthenticationException: 'Authentication failed.'

I have noticed that the authToken.Account is null and that's why I'm passing the account name in by the string username. Also it seems I have to use the .default scope as anything else causes an error on AcquireTokenForClient as per this question.

Any ideas what I'm doing wrong here?