capitilize the first word in a sentence

There is no setting in Apple Mail to capitalize the first word in a sentence. There is a setting in the System Preferences/Settings, and in different panels, depending upon your unmentioned operating system version — that enables capitalization of the first word of a sentence. Applications are then supposed to detect that and perform the expected capitalization.

For macOS Sonoma 14.4.1, even with that setting enabled in System Settings > Keyboard > Input Sources panel, the Mail program ignores the setting. It makes no difference whether one quits and restarts the Mail application, or reboots the Mac, Mail continues to ignore that System capitalization hint.

That means the old school shift key solution to capitalize the first word of sentences in Apple Mail.