Two days after installing Leopard Apple Mail started to crash within the first 10 seconds after opening the app. Apparently the error occured after using iSync with my Nokia N95 for the first time. similar behaviour is reported on Apple’s support pages. Toying with iSync Preferences didn’t work. Deleting these two files (com.apple.mail.plist, com.apple.mail.searchhistory.plist) from the User/Library/Preferences folder apparently did. Your Inbox might be lost, but if you are sporting an IMAP account none of the actual messages will be lost. Hope this helps.