I will assume you are using Classic Outlook. If not, see below.

AutoCorrect Options -> Capitalize first letter of sentences
Autocorrect, auto capitalize, and auto format your messages - Microsoft Support

But it's stupid. Doesn't know the difference between an abbreviation and the end of a sentence. Unfortunately it's all or nothing.
How do I change/turn off the feature of when typing a period then a space, the next letter is auto-capitalized? - Microsoft Q&A

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What version of Outlook do you have?
Please see this Microsoft Support article and reply with what platform (Windows, Mac, Android, iOS) and version of Outlook you are using.
What version of Outlook do I have? - Microsoft Support

Or if you access Outlook On the Web (Error) via a browser, what platform (Windows, Mac, Android, iOS) and browser (Chrome, Edge. Safari, Firefox) you are using.