Several Internet Mail providers are rolling out support for OAuth2 user authentication. Starting with version 1.5.0, Squirrel supports user authentication with OAuth 2.0 (Login with XOAUTH2). Before using OAuth 2.0, you will need to setup the OAuth 2.0 authentication with your Internet hosting provider to obtain an OAuth 2.0 access token.
Here are some links to help you in setting up the OAuth 2.0 login with your Internet mail provider.
Microsoft published an article on how to authenticate a connection using OAuth:
Google wrote in an article, that they will be limiting access to G Suite thru OAuth 2.0 in the future. We have not yet found an article with an easy to understand setup for OAuth 2.0. But here are some links which might be helpful: