In organizations where 2-factor authentication is enabled,

Some applications ( Skype, external apps ... ) that can't require 2FA, need a special app password


How to generate an application password:

  1. Login to portal.office.com
  2. Click on your account icon
  3. Go to the Security basics page and sign in to your Microsoft account. 
  4. Select More security options.
  5. Under App passwords, select Create a new app password.
    A new app password is generated and appears on your screen.
  6. Enter this app password where you would enter your normal Microsoft account password in the application.


Source: 

https://www.dcccd.edu/resources/online-services/password/pages/set-up-mobile-for-email.aspx