When receiving "Relay Denied" errors you can use the following instructions to set up Thunderbird (or Mozilla Suite) to use authentication with your SMTP server:
1) Go to "Tools -> Account Settings -> Outgoing Server (SMTP)".
(If using Mozilla Suite: go to "Edit -> Mail & Newsgroup Account Settings -> Outgoing Server (SMTP)".)
2) Select the server and press the Edit... button.
3) Check the "Use name and password" option.
4) Add your username. Your username (without the @mailsnare.net) and password.
5) Click "OK".