Microsoft is currently disabling Basic Authentication for e-mail protocols for security purposes but some software/devices still rely on Basic Authentication to connect.
For detailed information regarding this change, please check the Microsoft article:
In this case you still have an option to enable Basic Authentication for required protocols using the steps mentioned below.
Please note that Microsoft has a good reason to deprecate Basic Authentication and enabling and using Basic Authentication will have security implications.
If you want to re-enable a protocol that is disabled for Basic Auth, or want to see what protocols are disabled, follow the steps below:
Please navigate to help and support (Headset blue icon) in Microsoft admin center and then Type Diag: Enable Basic Auth in EXO, push enter and click on Run Tests. Once you click Run Tests, the tool will check your tenant settings to see if Basic Auth is disabled for any protocols and then display the results. If Basic Auth is not disabled for any protocols this will be displayed. But assuming Basic Auth is disabled for one or more protocols, a list of protocols will be displayed. For the protocol to have enabled Basic Auth choose the protocol and acknowledge the update setting. See sample below:
Security defaults should be disabled because this blocks legacy authentication, disable this at your own risk and make sure you at least set MFA for your admin accounts
Uncheck Modern Authentication in portal.office.com (if the option is available)
Settings => Org settings => Modern Authentication =>uncheck IMAP4.
Enable IMAP for the desired mailbox/user:
This should do the trick, if it doesn’t please let us know and we can register a Microsoft support ticket.