Mail freezes, will not open due to Messages Conflict.

OS 10.10.3

MacBook Pro, i7 tons of memory SSD

So, I'm posting this because I figured out what was causing the problem and how to deal with it until Apple fixes the system problem, or somebody figures out how I can permanent-fix it locally.

You may have experienced this - If you attempt to open Mail with Messages running, the Mail screen is mostly empty of content and unresponsive. It's maddening and this has been happening to my Mac for a few months now.

In the Support Community, I found, as you probably did, a clever method of restarting Mail that required trashing Library files like "envelope index" and that works, but you have to find those files and wait for the mailboxes to rebuild.

I just confirmed that the cause of the Mail freeze was a conflict with Messages where if Messages is running when I attempt to launch Mail, Mail freezes.

To launch Mail properly, simply confirm that Messages is not running before launching Mail.

If Messages is already running and Mail freezes when you attempt to launch it, resist the Windows-style urge throw your Mac through the wall. Force Quit Mail; Quit Messages; Launch Mail; Launch Messages; use your Mac like a human being using a Mac.