Okay I had beta 3 installed and mail wouldn't even open because it kept crashing. Tried everything that was recommended to get it fixed but no dice.
Install Beta 4 and mail now works.....kinda.
I tried sending somene an email and the compose window keeps flashing on and off (visible to not visible) about once a second without letup until I kill the process. So I then tried to clean out the indexed folders in the Library/Mail directory.
Mail came back and imported all of the messages and rebuilt the indices like you would expect. Tried to send the same mail again (it was recovered) and same thing..... flashes on and off until I kill the process.
I know the guys at Apple are working hard but I don't remember the Yosemite beta being nearly this buggy at all. But I'll document the issue in Radar and see if it gets addressed.
