Apple's latest OS X upgrade is causing problems with many users, causing Apple's email client to crash unexpectedly.
This, as you can imagine, is causing nothing less than outrage by those Mac users experiencing the fault.
The Mac OS 10.5.6 update seems to have caused the problems with only those users running the latest Leopard OS.
Archive and Reinstall
Apple's support bulletin notes that Mail's problems are due to "outdated third-party Mail plugins" or a "copy of Mail that wasn't updated properly."
So if you cannot fix the problem by running Software Update or fixing your outdated Mail plug-ins then Apple suggests you back up all your files, do an Archive and Reinstall of Mac OS X 10.5 and then re-run the update, which should solve the problem and stop you tearing more of your hair out.