I have four outlook.com mailboxes and two gmail mailboxes registered in the Mail application. For several years, the Mail app has worked fine with these accounts.
After applying the "January 28, 2020-KB4534132 Cumulative Update for .NET Framework 3.5 and 4.8 for Windows 10 Version 1903 and Windows Server 1903 RTM and Windows 10, version 1909 and Windows Server, version 1909" update, all four of the outlook.com mailboxes have disappeared from Mail, and only the gmail mailboxes remain. This is grossly unacceptable behavior. Running the "January 28, 2020—KB4532695 (OS Builds 18362.628 and 18363.628)" update leaves the mailboxes still missing.
When I look in "Settings - Accounts - Email & accounts" all six of the mailboxes are still registered there.
What the hell happened, and how do we have to recover from this error?