@Ben Ah I see. Sorry, I had interpreted your comment slightly differently.
You could probably cover a high percentage by allowing gmail, hotmail/outlook (I couldn't find the offical support document about it with a quick search) and protonmail domains though.
Indeed. Hopefully that is something we can consider as we continue to build out Watchtower.
If you do, please add FastMail to that mix.
You could always allow users to add additional email addresses for Watchtower checking, ensuring that ownership is correctly verified for each email address. Of course, you would still want to allow 1Password login with only the Primary email address.
It's certainly a possibility.