Using Modern Auth with Legacy Protocols
If legacy authentication is globally disabled in Microsoft 365, basic authentication for protocols like POP3, IMAP, SMTP, and ActiveSync will no longer work. However, some protocols can still function if configured to use modern authentication (OAuth 2.0).
POP3/IMAP: These protocols rely on basic authentication by default, which will stop working if legacy authentication is disabled. However, specific third-party clients or custom applications may support OAuth for these protocols. You must ensure that these clients are appropriately configured to use modern authentication.
SMTP AUTH: Similar to POP3/IMAP, if basic authentication is disabled, SMTP using basic authentication will no longer work. But, SMTP AUTH can still function if it is configured to use modern authentication (OAuth). In most cases, you will need to update your application's configuration to leverage OAuth instead of basic authentication.
ActiveSync: This protocol does not support OAuth. If legacy authentication is disabled, ActiveSync will stop working because it can only use basic authentication.