mail.app no longer working (OS 10.6.8)

Apple MacBook 2G Hz Intel Core Duo 2008 (black)

OS 10.6.8

Mail.app not opening.....wrong version! There has been no advice on what to do/support or explanation.

For a few months I have no longer been able to open Mail.app.

I have tried a backup older version I had, and this also does not open.

Below is the message I get when trying to open Mail.app vers 4.5 which I had download in March 24th 2011 and subsequently used.

What has happened to Mail.app? that I can no longer use it? And what can be done?