First, please change the title of your thread to "
A new version of Mail doesn't work in Snow Leopard" or, better yet, "
Version 4.5 of Mail won't work in my Snow Leopard
10.6.8."
Reason? 4.5 is not the new version of Mail, that would be 6+
The first thing I'd do would be to reinstall the version you previously had. It should be easy to find in Time Machine. Go back to just
before the date you installed the version that's not working.
Did you use the 10.6.8 Combo Updater? If not, get it <
from Apple> and see if it doesn't automatically install the correct version.
Last but not least, what is the version of the Mail app that is in your Applications folder? Just do a command-i on it and the Info window should pop up and show you that. If you're in Column view, you don't even have to do the command-i part, just select the app and its info will show in the next column. It may be the 3+ version that should have been updated when you installed SL. At least the Combo version of the installer will do that.