This happens when I archive an IMAP mailbox from Apple Mail as an mbox and then reimport it again under a new account:
Process: Mail  Path: /Applications/Mail.app/Contents/MacOS/Mail Identifier: com.apple.mail Version: 3.3 (924) Build Info: Mail-9240000~1 Code Type: PPC (Native) Parent Process: launchd  Date/Time: 2008-06-11 21:06:57.158 +0200 OS Version: Mac OS X 10.5.3 (9D34) Report Version: 6 Exception Type: EXC_BAD_ACCESS (SIGBUS) Exception Codes: KERN_PROTECTION_FAILURE at 0x0000000000000010 Crashed Thread: 14
Apple Mail can’t even import what it archived. Madness!
Many years ago I was able to force an incorrectly formatted mbox into a correctly formatted one using ‘formail’. Not sure if the mbox created by Apple Mail was incorrectly formatted, but I gave it a try anyway:
formail -ds <archived.mbox >>fixed_archived.mbox
No luck; Apple Mail still keeps crashing.