If you get an error stating 'Relaying Denied' when trying to send email then you need to ensure that you have the 'My server requires authentication' option ticked in your email software settings.

The Email Tutorials section of this helpdesk has guides to setting up your email client and ticking the correct authentication box, to solve any 'relaying denied' errors.

Outlook Express Setup Guide:

http://support.inchberryhosting.com/KB/a5/setting-up-your-email-in-outlook-express.aspx

Following the above will resolve this error for you.