To see the status of your accounts to check for problems, go to the Settings charm then click Accounts:

If there is an issue with an account you will see a message about an incorrect password or inability to connect.

If you have second factor authorization (e.g. app-specific passwords) you will need to enable it on the web site of your email provider.

Tip: Try logging in to your account through a browser to verify there isn’t a problem with your account. In some cases providers will temporarily block an account for activity from a new device, expired passwords, or other additional verifications.