This seems to have worked for me.
How did you figure it out?
Although on my phone (12.4.1) I went like this
Go to Settings > Passwords & Accounts > Mail
select your Microsoft email Account.
then check Account > Host Name
unter INCOMING MAIL SERVER
and change it to outlook.com and save the change.
What FaKre posted earlier I pasted here below
Nov 1, 2020 1:49 AM in response to Bulletvaldez
In my case the problem was, that the new certificate was only issued for outlook.com but I had „imap-mail.outlook.com“ set as incoming mail server.
Go to Settings > Mail > Accounts
select your Microsoft email Account.
then check Account > Host Name
unter INCOMING MAIL SERVER
and change it to outlook.com and save the change.