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 [318]
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 [79]

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.