New minimum Outlook for Windows version requirements for Microsoft 365

JRT_1984 and subhaschand 
We have started deprecating use of Basic authentication in M365 Exchange Online, perhaps you got affected by that. This is our announcement related to that initiative, please take a look: Exchange Online email applications stopped signing in, or keep asking for passwords? Start here. - Microsoft Community Hub.

And here's a blog post: Enabling Modern Auth for Outlook – How Hard Can It Be? - Microsoft Community Hub with steps to work thru to ensure you can still use Outlook 2013 in M365 EXO to connect to your mailbox, using Modern auth. You may need to enable Modern auth at your org-level and create a reg key, take a look at the instructions.

Outlook 2013 is still supported to connect to mailboxes in Exchange Online (until April 2023, see: End of support for Office 2013 (microsoft.com)), you just need to make sure you are using the latest build. To ensure that, please install the latest available update over here: Latest updates for versions of Office that use Windows Installer (MSI) - Office release notes | Microsoft Learn

Let us know if you have any questions after going thru the linked articles.