Microsoft recently announced that 99.9% of the attacks on Office 365 credentials can be stopped by enabling multi-factor authentication (MFA) Third-party tools that plug into your online applications might no longer work. If you disable legacy authentication on Outlook 10, it wont be able to connect to Office 365. If they use legacy authentication, they are basically using IMAP, POP, SMTP and other older protocols to connect. Enforced MFA is a good thing, because you need to demand that vendors stop using an old insecure method.”]