Yosemite - Mail not working After installing Yosemite, Clicked on "Mail" it said, "Installing, will take a few minutes" when that finishes, it says "optimizing mailboxes" and then immediately crashes.

Tried deleting accounts, and then adding them back.  Tried rebooting.  Still the same happens each time.  Can you offer helpful suggestions?
A couple of things to try:

Go to System Preferences > Internet Accounts > Turn off any email accounts, and try launching Mail again. 

Open Terminal.app, and paste or type:     ~/Library/Containers/com.apple.mail/Data/Library/

Find the Saved Application State folder, drag it to Trash, and try opening Mail. Finder will re-create the folder, and this should clear out anything that may have been corrupted when you updated OS X.