Then, a preferences window will apear. You should select "Sending (SMTP)" tab. The window will then look like on the image below.
If you want to use SMTP AUTH with the same login/password pair as for POP3 then select "Login using my POP3 settings for username and password". If you want to use anothe pair, then check "Login with username" and fill the 2 fields with your login and password on SMTP server.