(Screenshots in this document are from the Windows version. These instructions apply to all OSes however.)
To enable authentication for outgoing mail in Thunderbird 1.5:
Open Thunderbird
Select Account Settings... from the Tools menu. (From the Edit menu in Linux)
Click on the Outgoing Server (SMTP) account from the list on the left, then click the Edit... button on the right.
Check Use name and password under Security and Authentication
Enter your UW-Platteville NetID in the User Name field, then click OK