This terrible update just bricked my entire Apple Mail app.
Leave it to Apple to screw up every single update that they ever release.
I just updated macOS from 10.3.2 to macOS 10.3.3, simply to fix Apple's OTHER BUG of Messages appearing out of order... and lo and behold, after updating, Apple Mail doesn't work anymore.
Apple Mail will launch, but when trying to compose a new message, nothing but a blank compose window shows up on the screen. Nothing else responds in Apple Mail, either. The entire program just sits there, completely unresponsive.
Playing devil's advocate, I even allowed Mail to sit unattended for 3 hours on its own... and after coming back 3 hours later, absolutely nothing. Same exact thing.
Force-quitting Mail and relaunching it doesn't do a thing... same exact problem.
Restarting the Mac doesn't do a thing... same exact problem.
Gee, thanks, Apple! Now I have a non-working email program, thanks to you!!
Great job on your continued downward spiral into irrelevance in the technology world.