Error when trying to send mail: "553 sorry, relaying denied from your location [] (#5.7.1)"

Email > Troubleshooting email problems and errors
This error indicates that your email client is not configured to use SMTP Authentication. To protect against unauthorized access to our SMTP servers, users are required to authenticate with a valid email account.

See the following articles for information on how to configure your email client to use SMTP Authentication

Outlook Express
Outlook XP/2002/2003
Outlook 2007
Mozilla Thunderbird
Mac Mail
Mac Entourage