To reinstall Mail:
1. Insert the Snow Leopard 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 to bring iCal up to the most recent version.
I've always known about this way, but I had hoped that there was an easier, safer way. Frankly, the idea has always frightened me. I've been afraid of losing everythng in the attempt. However, since I am using an external hard drive, since no one has supplied an easier, safer way, and because you have laid out the steps so clearly and thoroughly, I'm going to do it. Wish me luck.
I finally got the nerve to try following your advice, and although it didn't work because I forgot to open my computer in its Snow Leopard side, I finally was successful. It was easy. All I had to do was (1) open computer in Snow Leopard, (2) use "Spotlight" to find Snow Leopard." (3) Click on MAIL. That's it.That's all I had to do. Now I have MAIL in both the Snow Leopard and Mountain Lion sides of my computer, and now I see the way to recover other applications that I had lost/misplaced. Nevertheless, I do thank you for your advice (I'm sure it would have worked); in fact, if I hadn't started to follow your instructions, I probably woudn't have ultimately been successful.