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.