With the release of Indigo 2021.1, we removed the old built-in email capabilities and replaced it with the Email+ Plugin which is based on the Better Email plugin from forum user @FlyingDiver (many thanks for his generous contribution). The 2021.1 installer will migrate both old settings and actions from the built-in email and any devices and actions from Better Email if it was installed.
If you are using a version prior to Indigo 2021.1, you should be using the Better Email plugin since it's significantly more reliable than the old built-in email. We will leave that plugin in the Plugin Store for users of older versions of Indigo.