I have noticed that Outlook 365 app does not tell a user who uses 2 step authentication on the Outlook.com to get and provide the application password and not a password of the account the user tries to add to the Outlook 365 app.
It took me two weeks to guess this. Please add some prechecking then adding accounts having such 2FA or at least give o pop up letting the user to know that if 2FA is being used, a password not from a web login has to be provided.

Outlook 2016 should allow users to authenticate using OAuth2. This is the only supported option for connecting a Gmail account, and it's a lot more secure than many other authentication solutions. It would make life a lot easier, because many webmail services utilise OAuth.

Most of my mail accounts (Outlook.com and GMail) have two factor authentication set up. It's really annoying to add them to Outlook 2016. I have to generate a app password on each of the accounts, and use them to log in.

Your mobile apps (iOS and Android) use the "native" service authentication (redirect me to the site, I log in, authorize the app and I'm good to go). I find it a much better way to log in, since I don't need to go to another site and generate a password. All I need is my phone.