How to validate smtp credentials before sending mail?

I need to validate the username and password set in an SmtpClient instance before sending mail. Using this code:

SmtpClient client = new SmtpClient(host);
client.Credentials = new NetworkCredential(username,password);
client.UseDefaultCredentials = false;

// Here I need to verify the credentials(i.e. username and password)
client.Send(mail);

How can I validate whether the user identified by the credentials is allowed to connect and send a mail?