On March 7, 2018 the Microsoft Exchange Team announced that on October 13, 2020 it would stop the support for Basic Authentication (also called Legacy authentication) for Exchange Web Services (EWS) in Exchange Online (EXO), the version of Exchange offered as a service part of Office 365. EWS is a web service which can be used by client applications to access the EXO environment. The team also announced that EWS would not receive any feature updates anymore, and suggests customers to transition towards using Microsoft Graph to access EXO.
One and a half year later, on November 20, 2019 the Exchange Team also announced to stop supporting Basic Authentication for Exchange ActiveSync (EAS), Post Office Protocol (POP), Internet Message Access Protocol (IMAP) and Remote PowerShell on October 13 2020 as well. Authenticated Simple Mail Transfer Protocol (SMTP) will stay supported when used with Basic Authentication.
Instead of supporting Basic/Legacy authentication Microsoft will move towards only supporting Modern Authentication for most of the methods used to connect to Exchange Online.Read More