How to access Exchange Online using IMAP, POP or SMTP legacy protocols

The basic authentication turnoff for Exchange Online is now in effect. October 1, 22 marks the point when Microsoft begins to disable seven email connectivity protocols for Exchange Online. It will take time for Microsoft to process all tenants, but eventually those wishing to use protocols like POP3, IMAP4, and Exchange ActiveSync will have no choice but to use modern authentication. In other words, a username and password won’t be enough.

Still there are many companies having applications that use POP3 or IMAP4 and Microsoft’s solution now is to allow customers to create Azure AD registered apps and assign the necessary permissions to allow the apps to use IMAP4 or POP3 to interact with mailboxes. Below figure shows the assignment of the IMAP.AccessAsApp permission to an app. The equivalent permission for POP3 access is POP.AccessAsApp.

Organizations can use the OAuth authentication service provided by Azure Active Directory (Azure AD) to enable their application to connect with IMAP, POP or SMTP protocols to access Exchange Online in Office 365. To use OAuth, following are the steps,

1. Register your application with Azure AD.
2. Get an access token from a token server.
3. Authenticate connection requests with an access token.

Check detailed steps to configure the above in below mentioned Microsoft's article:

https://learn.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth