Most email services will require you to generate an app specific password for Big Mail. This is so third-party apps like Big Mail don't see your password.
Generating an app specific password is quick and easy. Select your mail provider from the ‘Add an account’ screen, and click “Generate Password”. You’ll then be taken to your mail provider to generate your app specific password. Once done, simply copy the password back into Big Mail.
Whilst Big Mail doesn’t support Two Factor Authentication (2FA) directly, as long as your mail provider allows you to generate an ‘App password’, you’ll be able to sign in with Big Mail.