Have you searched for Mail using Spotlight? If you have completed a thorough search of your HD and are sure that it is no longer in the Applications Folder use these steps to reinstall Mail.
To reinstall Mail, you must have a system installation disk. Here is the procedure for OS X 10.6, or 10.5.
1. Insert the Installation disk into the computer.
2. The disk will appear on the desktop.
3. Double click on the "Optional Installs" folder.
4. Double click on the "Optional Installs.mpkg."
5. Click on (Continue) in Introduction.
6. Click on (Continue) in Software License Agreement.
7. Click on (Agree) to agree to the terms of the software license agreement.
8. Click on the disclosure arrow ▶ next to Applications.
9. Select (✓) Mail.
10. Click (Continue).
11. After Mail has been reinstalled, download and install the Mac OS X 10.6.8 Update Combo v1.1, for Snow Leopard or Mac OS X 10.5.8 Combo Update for Leopard to bring Mail up to the most recent version.
I am not well versed in obtaining older versions of OS X. The installation disk will not be available through Apple, but there are other sources available which may or may not be reliable. The model of your computer must also be considered when obtaining OS X installation disks and considering upgrade options. I recommend that you post your question in the Mac OS X v10.5 Leopard: Mac OS & System Software: Apple Support Communities Forum.
* Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor
minimum system requirements
* 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)
* DVD drive for installation
* 9GB of available disk space (I say 30GB at least)
Classic/OS9 Apps no longer supported.
Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...