Hi,
Your code snippet is working with gmail smtp.
Assuming that you are working with an exchange server, you can change your code to:
Before:
smtpClient.Credentials = new System.Net.NetworkCredential(sender, password);
After:
smtpClient.Credentials = System.Net.CredentialCache.DefaultCredentials;
with gmail smtp this solution won't work.
probably you will need to impersonate the user account before make the call, otherwise the credentials will be from the asp.net process.