It probably does not matter about whether the domain is a .onion one, if I am
honest.
I should point out that communicating with any of your associates from the
Tor account would probably reveal your identity. Therefore, I don't think
that if you are just going to use it for regular emails that you should use
an email account solely on Tor. You could get an account using a Tor-friendly
email provider, and use Torbirdy to hide your location, and use GPG to
encrypt the messages as much as possible, keeping in mind that most people do
not use GPG, and probably never will. I think you should look at this page:
https://www.fsf.org/resources/webmail-systems I have used Posteo, which costs
1 EUR a month but I sent it in cash. Other options are OpenMailBox, which is
gratis.
You should only get an account that you solely use on Tor if you want to take
part in anonymous activism. This does not mean less important things
(compared to getting rid of authoritarian governments) like promoting free
software. This means things like whistleblowing, trades union movements in
authoritarian states, and pro-democracy activism in countries like China.
Using such an account would be for anonymous blogs, and communicating with
other activists hiding their identity ove GPG, certainly not for silly things
like online shopping.
The point I am making is that the way that emails work means that using a
Tor-only email address to communicate with your associates inherently removes
anonymity. You have to be careful when using Tor and make sure you use it
correctly.
I do not know if you are an activist, but I will assume not. I think what
will be best for you is to make an account with one of the services listed on
the page, and to use TorBirdy if you want location privacy, but to keep in
mind that it will not be entirely anonymous as far as it goes. Use GPG as
much as possible: https://emailselfdefense.fsf.org/en/ (there are other
languages available, such as Russian, French, Spanish, Japanese and Turkish)
