Existing users, 
A workaround was posted to the Mail.appetizer mailing list yesterday. I don't have the message here on my work computer, but I believe the following is the correct procedure:
1. Ensure that Mail.app is not running.
2. Launch Terminal and enter these two commands:
defaults write com.apple.mail EnableBundles 1
defaults write com.apple.mail BundleCompatibilityVersion 3
3. Move the Mail.appetizer bundle from the folder ~/Library/Mail/Bundles (Disabled) back to ~/Library/Mail/Bundles.
4. Launch Mail.app -- all should be well again.
Hope this is right -- going from memory and using an XP machine at work so I can't verify. At worst it won't work but won't do any damage. :-)
Mail.appetizer
Have a look at your incoming email without switching to Mail.
Version: 1.3b1
Leopard fix / workaround
Feedback Type: Troubleshooting Report
Contributed by: brossow Tuesday, October 30 2007 @ 10:52 AM PDT
Product Platform: MacOSX
Used Product For: Over One Year
Recommend Product: YES
System Info:
Comments
Leopard fix / workaround - hektisk
This workaround allows mail to open with appetizer ostensibly installed (it has its own tab in 'Preferences", etc) but appetizer actually doesn't work. 'Enable mail.appetizer' is checked, but nothing pops up when I get a new mail. Is this happening to anyone else?Thursday, November 01 2007 @ 06:30 PM PDT
Leopard fix / workaround - hektisk
This workaround allows mail to open with appetizer ostensibly installed (it has its own tab in 'Preferences", etc) but appetizer actually doesn't work. 'Enable mail.appetizer' is checked, but nothing pops up when I get a new mail. Is this happening to anyone else?Thursday, November 01 2007 @ 07:00 PM PDT
Leopard fix / workaround - wiktor.klatkowski
Hi, have you solved the problem? Most propably I have the same problem. Pls let me know. Thanks, WiktorFriday, November 16 2007 @ 09:45 AM PST
Leopard fix / workaround - raphaeladidas1
In step 2, what were the values of those parameters before?Wednesday, November 07 2007 @ 08:25 PM PST
Leopard fix / workaround - carthage9
The workaround works perfectly, so glad to have this small but invaluable tool working on Leopard again!!! Thanks :)Monday, December 24 2007 @ 01:10 AM PST
Leopard fix / workaround - julien.delprat
I doesn't work for me neither. I can see the preferences of mail.appetizer in mail.app but nothing pops when a new message arrive. Can someone help me ?Friday, February 15 2008 @ 12:00 AM PST
Leopard fix / workaround - Diceke
Works!Thanks.
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Wednesday, October 31 2007 @ 08:18 AM PDT