Simple email client with webextensions - is it possible

I wonder if would be possible to make a simple email client that fetch mail with the pop3 protocol?

The fetched mail could be stored in an indexed db data base and processed in various ways.

Are there any extensions that could serve as examples?

You can only do HTTP transfer without a native companion application, so it’s not possible to do POP, SMTP or IMAP out of the box.

Thanks.

But you could do it with a native companion application?

Where can I find more information on the options with a ‘native companion application’?

This describes how the messaging with the native application works: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/Native_messaging