Activating an external email address when using two-factor authentication
From Planfix
Suppose the external mail server you are using has two-factor authentication enabled. In that case, additional steps are required to connect external email addresses to third-party applications, as regulated by the service provider. Below, you will find detailed instructions for several popular email services:
If you are using the services of another provider, please contact their support service for instructions on how to connect third-party applications.