If you receive a relaying denied error while trying to send email you are likely trying to send email through a mail server while not connected to that ISP's network. For example, you might be trying to send email through your companies outgoing mail servers while connected to your internet connection at home. The best solutions are to either use smtp authentication so you can send through your preferred outgoing smtp mail servers from any network, or use the outgoing mail servers of the ISP that is providing your current internet connection.
