Email Error: SMTP Error: The following recipients failed--please Help

[sogoeo]

In zen-cart admin---tools-----send mail.
When I tried to send mails to customers, the following errors cccured:

Email Error: SMTP Error: The following recipients failed: [email protected].

Below is my config in admin:

Title Value Action
E-Mail Transport Method smtpauth
E-Mail Linefeeds LF
Use MIME HTML When Sending Emails false
Send E-Mails true
Email Archiving Active? false
E-Mail Friendly-Errors false
Email Address (Displayed to Contact you) [email protected]
Email Address (sent FROM) [email protected]
Emails must send from known domain? Yes
Email Admin Format? T

So what is the problem and what do I need to set them to?
Please help. Thanks!