https://www.bleepingcomputer.com/news/security/cisa-warns-orgs-to-switch-to-exchange-online-modern-auth-until-october/

CISA has urged government agencies and private sector organizations using Microsoft's Exchange cloud email platform to expedite the switch from Basic Authentication legacy authentication methods without multifactor authentication (MFA) support to Modern Authentication alternatives.

Basic Auth (proxy authentication) is an HTTP-based auth scheme used by apps to send credentials in plain text to servers, endpoints, or online services.