In October 2022 Microsoft disabled some authentication protocols for their hosted Exchange Server email service. This means some clients like Outlook 2013 will not connect unless it uses Modern Authentication. This is not enabled by default, and it needs to be enabled and Outlook needs to be forced to use it. Here are some tips on how to fix this issue:
- Overview of issue: Basic Authentication Deprecation in Exchange Online – September 2022 Update
- The Microsoft Support and Recovery Assistant can most probably fix this
- This is the direct link to fix Outlook connection issues
- If you want to fix the Outlook issue manually, you may find that some settings that you need to change are greyed out like this. This is how to fix it: Microsoft Exchange Mail Profile Security Tab Greyed Out