I recently reinstalled OS 10.6 then did an update and was updated to 10.6.8. I am not able to send mail from my mail account. I am able to receive just fine and my server is working fine. My mail preferences account shows smtp.core.com (offline)

I have this problem also! I have added a new account different email address and that works fine but I would still like to use my original email address. There is nothing wrong with original address as it works fine sending mail from my ipad and also from msn just not from mail!!

Mac OS X v10.6: Mail.app won’t open, or "You can't use this version of Mail…" alert after installing Security Update 2012-004:

http://support.apple.com/kb/TS4424?viewlocale=en_US&locale=en_US

Fellow user Grant Bennet-Alder offers this solution:

Some users have reported this problem if the Mail Application has been moved out of the top-level /Applications folder, or duplicated in another location.

When the Security Update is done, the old version of Mail is disabled.

The solution has been to:

1) make certain Mail is in the /Applications folder

2) There is no other copy anywhere else.

3) Once steps 1 and 2 have been done, Manually download and re-apply the Security Update (2012-004) by hand.

Security Update 2012-004 (Snow Leopard)

If the Mail.app has been LOST, it can be re-installed by applying the 10.6.8 version 1.1 combo update. But this update is quite large and it is usually not necessary:

Mac OS X 10.6.8 Update Combo v1.1