I’m sorry to say it, but Outlook is over the hill. Outlook used to be the go-to email client for everyone. Everyone that still tries to use it as their client without some form of Microsoft back-end is in denial.
There are some clients, such as Thunderbird (https://www.mozilla.org/en-US/thunderbird/), that do great with agnostic email servers.
My suggestion is if you want to pick an email client, pick it, and then use the accompanying server. You have Microsoft with Exchange (both on-premise and cloud), you have Apple Mail, and you have the leading open-source combo today that is Zimbra (which also has a paid option if you MUST use Outlook).
But please, whatever you do, don’t try to cobble together something that doesn’t fit.