I have successfully installed XAMPP as directed in the Zen Cart manual. Mercury Mail is working and sending messages too.

I installed Thunderbird and set up the email accounts as directed in the manual on pages 42-44. I tried sending an email message from [email protected] to [email protected]. It doesn't work. I get this error message:
"An error occurred while sending mail. We do not relay non-local mail. Sorry. Please check the message recipients and try again."

The outgoing mail server for all the email account properties says:
storekeeper - mail.example.com (Default)

I run Microsoft Vista on this computer.

Any help would be greatly appreciated.