To enable authentication for outgoing mail in Mac OS X Mail.app:
Open Mail.app
Select Preferences... from the Mail menu.
If it is not already selected, select the Accounts icon.
Verify that all of your user information is correct. Click on the Server Settings... button.
In the Outgoing Mail Server field, verify that the address is smtp.nw.uwplatt.edu
Change Authentication to Password
Enter your UW-Platteville NetID in the User Name field, then click OK
Click Save. You will be prompted for your password....
Enter your password and click OK.