Hi, I had a several hour session with you in December at which time you recommended that I purchase and install the Leopard operating system.  I finally did, as I wanted to wait until the new ILIFE was out as well.  It has completely shut down my email, and I can now not even open mail from my computer.  How am I supposed to deal with this?  Do you have a solution?  Everything else seems to be working.
Thanks,

I would first try running all updates. There may be crucial updates that will fix the issue without having to do anything.

This has happened to others before. Here are a few suggestions:

If it’s related to rules, then tossing the rules plist file could solve it. It’s located at ~/Users/Yourusername/Library/Mail/MessageRules.plist

You might also toss out the backup rule.plist file as well. Apple describes it this way:

1) Force quit Mail if necessary: From the Apple menu, choose Force Quit, select Mail in the Force Quit window's list of applications, then click "Force Quit".

2) If the issue persists after relaunching Mail, follow these steps while Mail is not running:

3) From the Finder's Go menu choose Go to Folder. Go to this location (type or paste the following, then click Go): ~/Library/Mail

4) Move the items named "MessageRules.plist" and "MessageRules.plist.backup" to another location, such as your desktop.

5) Open Mail. Note: At this point you can either set your Junk Mail settings and re-create your Mail rules manually in Mail preferences, or follow the next two steps (if you re-create rules and Junk settings, do not perform the following steps).

6 ) Quit Mail.

7) Place the items named "MessageRules.plist" and "MessageRules.plist.backup" back in the ~/Library/Mail folder (replace any copies that were added to ~/Library/Mail when you opened Mail).



You can also try a more serious fix. Using terminal (only if you feel brave and advanced enough to use terminal):
Caution!!!! Using terminal can severely destroy your system. So proceed only if you feel confident and have an understanding of Terminal.

Utilities>Terminal:

defaults remove com.apple.mail JunkMailBehaviorHasBeenConfigured
defaults remove com.apple.mail JunkMailHeaderFlags
defaults remove com.apple.mail JunkMailInfoPanelHasBeenShown
defaults remove com.apple.mail JunkViewerAttributes
defaults remove com.apple.mail LastTimeJunkWasVisited
defaults remove com.apple.mail NumberOfMessagesMarkedAsJunk
defaults remove com.apple.mail NumberOfMessagesMarkedAsNotJunk
defaults remove com.apple.mail UseJunkMailHeaderFlags

Then press Return and quit Terminal.

Another solution. You may have installed a thrid party plugin into mail. Apple provides instructions here on how to deal with it:
Mac OS X 10.5.6: Mail unexpectedly quits