Yosemite upgrade causing Mail to crash
Yosemite mail is horrible, if you have a large inbox it just kills an older machine.  I have a 3ghz mini with 16gb of ram and it STILL crawls.  I am on day 3 of it saying "incoming messages" and slowly the count grows larger and larger. Two apps is all I ask for, Safari and Mail, really people FIX IT.

On my 2009 27" imac I had to restore from time machine back to mavericks.  If you start a restore it will show you what os version was backed up.  Just know it erases your drive so save all of your newer files elsewhere.