I seem to get more queries about the E-mail notifier than any other feature. Usually, it comes down to not filling in all of the fields, or setting the wrong authentication.
So the first thing I'd recommend is checking that both the To and From address fields are filled in with valid e-mail addresses.
If that doesn't help, check that the Outgoing Mail Server is correct in the E-mail Transport Options panel - if you use .Mac, make sure you enter just "smtp.mac.com" (without the quotes), and not the colon and username following.
If still not working, try each of the Authentication options in that panel, with the appropriate User Name and Password entered. Also try turning SSL off. It's not always easy to tell which will work.