Lost the Mail program

Take your Mac Pro's OS 10.5.0 Leopard installer DVD, navigate into the Optional Installs folder, launch the optional installs package installer, and choose the Mail program to (re)install. When complete, get the OS 10.5.8 combo installer from http://support.apple.com/downloads/MacOS_X_10_5_8_ComboUpdate and reapply it.

If, in your migration process, all the old Tiger (or older) applications were copied over, you may want to just select everything to reinstall from the optional installs list (Address Book, Mail, etc.) so you don't run into the same problem with other apps.

Well, this is pretty easy. It's not like you're booting up from the installer DVD and reinstalling the OS or anything like that. You're opening up the installer DVD and double-clicking on the optional installers pkg installer. Not any harder than downloading something from macupdate.com and double-clicking on that installer. Pretty quick. Not like wasting an hour doing an OS install. While you're doing that, have the combo update download running in the background, if time is that critical. Reapplying the combo updater doesn't take very long either.