This is a package that contains the code given by UriHendler in the issue #332 of MailKit.
With this mechanism, you achieve a similar result as if you were to use the UseDefaultCredentials()
of the .Net SmtpClient.
using MailKit.Security;
var mechanism = new SaslMechanismNtlmIntegrated();
smtpClient.Authenticate(mechanism);
string authenticatedUserName = mechanism.AuthenticatedUserName;