SYMPTOMS
If you encounter error as below while sending emails from mail client
such like Microsoft Outlook, Mozilla Thunderbird , Windows Live Mail
etc , the error is most likely SMTP Authencation is not being turned on
on the mail client settings.

WORKAROUND
You could turn on SMTP Authentication on your mail client as here
Article ID: 2844, Created: June 21, 2011 at 8:25 AM, Modified: June 21, 2011 at 8:30 AM