For information about features offered by specific POP3 and IMAP4 client programs, see the documentation that's included with each application. The features supported by each email client programs vary. Users can use any email programs that support POP3 and IMAP4 to connect to Exchange Online (for example, Outlook, Windows Mail, and Mozilla Thunderbird). Disabling Basic authentication will block legacy protocols, such as POP and IMAP. To protect your Exchange Online tenant from brute force or password spray attacks, your organization will need to Disable Basic authentication in Exchange Online and only use Modern authentication for Outlook in Exchange Online.
For more information, see What are security defaults?. If you've enabled security defaults in your organization, POP3 and IMAP4 are automatically disabled in Exchange Online.
