Hi, @TV
Looks like the issue is more related to your outlook.com account, please kindly understand that the Microsoft Exchange tag here we mainly focus on general issues about Exchange and mail flow. It's suggested that you could post your case in dedicated forum for Outlook.com for more professional advice.
You can do the following: Set up an Outlook account on the iOS Mail app - Microsoft Support
Here is similar case for your reference: Native iOS mail client modern authentication. - Microsoft Q&A
Apple iOS Mail App Might Need Upgrade for Exchange Online (office365itpros.com)
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