Besides having installation problems, Apple also acknowledged problems with Mail.app for some users after installing the update for 10.5.6.

After updating to Mac OS X 10.5.6, Mail may unexpectedly quit. This can be caused by having outdated third-party Mail plugins installed, or by using a copy of Mail that wasn’t updated properly (you may need to reinstall the Mac OS X 10.5.6 Combo Update).

Apple also released a download to update the Mail.app properly. Let me know if you have any problems and whether it is solved.