UPDATE:
2 DWORDS to be Checked/Added to Registry -
1. HKCU\SOFTWARE\Microsoft\Office\15.0\Common\Identity\EnableADAL REG_DWORD 1
2. HKCU\SOFTWARE\Microsoft\Office\15.0\Common\Identity\Version REG_DWORD 1
Microsoft has now turned off basic authentication for Exchange Online.
Microsoft 365 Business, Exchange type email accounts will no longer work natively with Outlook/Office 2013 and below.
To fix, either upgrade office software from 2013 to newer 2016 or higher.
Alternatively follow the guide below to enable the email to work in Office 2013 -
Save the below text into a notepad document and Save the file with the file extension .reg instead of .txt in a location that's easy for you to find. For example, C:\Data\Office2013_Enable_ModernAuth.reg. Double click to open and click yes to accept the changes.
Text for notepad file:
Windows Registry Editor Version 5.00
[HKEY_CURRENT_USER\Software\Microsoft\Exchange]
"AlwaysUseMSOAuthForAutoDiscover"=dword:00000001
[HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Common]
[HKEY_CURRENT_USER\Software\Microsoft\Office\15.0\Common\Identity]
"EnableADAL"=dword:00000001
"Version"=dword:00000001