I'm assuming you didn't have a Time Machine backup. See if the combo updater will update Mail.
Combo updater: Mac OS X 10.6.8 Update v.1.1 - 1.09 GB
Have you considered upgrading to Mavericks? :-)
Thanks Dianeoforegon, now (after combo update) I have Mail 4.5, so something changed, but it still doesn't run... :/
No, I didn't have Time machine, nowadays I have Superduper Backup, which works fine for me, but Mail was deleted quite some time ago.
And unfortunately I can't leave Snow leopard, as I'm a filmmaker using Final cut pro, though I guess I'll have to look for a new solution to that soon, too.
I use SuperDuper as well. However your case is a perfect example of where Time Machine would have saved you. Just like a seat belt and an air bag protect you in different ways when driving, you need both Time Machine and a clone for full protection.
And unfortunately I can't leave Snow leopard,
You can run Snow Leopard server using a VM. You get the best of both worlds.
Installing Snow Leopard Server into Parallels
Mac OS X 10.6 Snow Leopard - Apple Store (U.S.) $19.99 + sales tax & shipping (call 1.800.MYAPPLE
Part no: MC588Z/A