If the smtp server requires authentication it even fails if the correct credentials are provided.
I’ve found the following user comments in the VS documentation:
„UseDefaultCredentials = false“ has to be set before setting the new credentials.
[http://msdn.microsoft.com/en-us/library/vstudio/system.net.mail.smtpclient.credentials(v=vs.100).aspx](http://msdn.microsoft.com/en-us/library/vstudio/system.net.mail.smtpclient.credentials(v=vs.100).aspx)
Changing the code in “SmtpDeliveryService.cs” and “MailSettings.ascx.cs” accordingly has solved the problem. Please includefix in the next release.
```
if (smtpAuthentictation)
{
client.UseDefaultCredentials = false;
client.Credentials = new NetworkCredential(smtpUsername, smtpPassword, smtpDomain);
}
```