Need to re-install mail.app

Restoring Lost OS X Files

If you erase or otherwise "lose" an OS X system application or utility (such as Mail or Disk Utility, for example) you can restore them without having to reinstall OS X.

Download the shareware utility Pacifist. Read the documentation. Use Pacifist to extract a fresh copy of the missing application from the file archives on your OS X Installer disc. The file archives are usually found on the installer disc in the /System/Installations/ folder.

After restoring the application you need to reinstall the OS X Combo Updater for the version of OS X you currently have installed. This assures the new copy of the application is updated to the current version.

Lastly, it's a good idea to repair permissions.

In Mac OS X 10.4 (Tiger), you don’t need Pacifist for that. Mail is certainly an option if you follow the instructions provided in the second part of the following article (under More custom reinstallation options):

Custom installs in Mac OS X 10.4

Note that Mail may appear grayed out in the installer, but will be checked if you check Applications there, and will be installed if Mail has been removed from the /Applications/ folder.

If the version of the system software on the Install discs is earlier than what you’re running now (and only in that case), then you must also reinstall the Combo Update for the type of computer and the version of Mac OS X you’re running, to bring Mail back in sync with the rest of the system, as Kappy said:

About the Mac OS X 10.4.11 Update
Mac OS X 10.4.11 Combo Update (PowerPC)
Mac OS X 10.4.11 Combo Update (Intel)

Additionally, if you had installed any Security Updates released afterwards that included updates for the application you’re reinstalling, as is the case with the latest Security Update and Mail, you’ll have to reinstall that as well:

About Security Update 2007-009
Security Update 2007-009 (10.4.11 PowerPC)
Security Update 2007-009 (10.4.11 Universal)

Actually, since Security Update 2007-009 already contains the latest version of Mail, reinstalling the Combo Update in this particular case is probably unnecessary if you’re going to reinstall the Security Update, but in general, if you follow the procedure just described, you won’t have to worry about which updates contain what exactly.