The following article details how to fix the error message listed below. The most likely cause of this error message is not having 'My Server Requires Authentication' selection checked in your email client.

        The message could not be sent because one of the recipients was rejected by
         the server.
        The rejected email address was 'email@email.com'.
        Subject 'test', Account: 'any account', Server:'mail.yourdomain.com',
        Protocol: SMTP, Server Response: '550 not local host 'todomain.com',
        not a gateway',
        Port: 25, Secure(SSL): No, Server Error: 550, Error Number: 0x800CCC79

To resolve this error message, please follow these steps:

1. Open your mail client.
2. From the Tools menu, select Accounts.
3. Select your mail account and click Properties.
   
4. From the Server tab check My server requires authentication.
   
5. Click Ok.

You should now be able to send your email successfully.