Mail app does not capitalize the first letter of a sentence. It never has, going back many versions. the Mail app in iOS on iPad and iPhone both do. Many word processors capitalize the first letter of a sentence. (Yes, I know how to use the Shift key.)
I wonder if the updated Mail app in El Capitan will? But I won't be surprised if it doesn't. Apple seemus to be stubborn about this or just ignores it.
No one here is prepared to talk about unreleased OS X software based on the established beta, and this support community guidelines. You are not talking to Apple here either, just international volunteers, not unlike yourself.
Go get the free WordService collection of OS X services from the OS X App Store. When it downloads, it installs useful services, that will need to be individually enabled in System Preferences : Keyboard : Shortcuts : Services. It has a service named: WordService:Initial Caps of Sentences. Just type in your mail message, select it, and run this Mail > Services > MailService:Initial Caps of Sentences. Instant auto-capitalization in the block of selected text.
The WordService application will appear in your Launchpad items. Its only purpose through its Help button is to launch Preview with a PDF description of what its services perform.
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Mail app does not capitalize first letter of sentence
