Mail app not opening on yosemite I recently upgraded to Yosemite on my Macbook Pro. For a couple of days, my mail app was working. Then suddenly it stopped working. 

I had to force quit the program and since then the app refuses to even open. The beach ball keeps rotating, and it continuously shows app not responding.
What is the solution to this?
My problem with Mail on Yosemite has been the same on three different Macs; new iMac with Retina display, new MacBook Pro with Retina display, and older iMac. Same issue. Half the time or more Mail freezes up upon launch, which required a Force Quit. Upon restarting, most of the time Mail would function OK, but occasionally hang again, always with the same error-- Application Not Responding. I've tried every fix on this thread and other website threads to get it fixed with nothing working.

Here's what I did to fix it.

I left the Mail Preferences window open to Accounts. That's it. Now, when I restart the Mac or it starts up automatically in the AM, Mail launches as per normal, but with the Preferences window open to Accounts. No problems since. None. Zero.

I have over a dozen email accounts, all IMAP, ranging from Apple's iCloud to Gmail to standard IMAP accounts, personal and business. Obviously, there's a bug somewhere. I came upon the fix by disabling all email accounts, re-enabling each, one-by-one, then doing a shutdown and startup. About five re-boots in I inadvertently left the Preferences window open to Accounts. No freeze on the next reboot. When I close Preferences, the freeze begins again.

It's a cheap fix but it took a long time to get to it.