If an email provider uses an .onion address, does the email exit their servers into the internet through a tor exit node? Could a powerful adversary track it?
It depends, there are a few scenarios.
Internal mail, from:
- The email never leaves the foo.com server(s), there's no way to intercept it without compromising the mail server(s).
Onion to onion, from:
- This is the same as internal except now either of the two servers would need to be compromised to gain visibility. It would need to be supported by both mail providers to work, which isn't common, I don't know of any that do.
External to onion, from:
- This is mostly the same as onion to onion, since the outbound mail from foo.com connects over tor to bar.onion.
Onion to external, from:
- Yes, this would traverse the internet in some way and would have to leave the Tor network.
- Onion to onion mail isn't widely supported, until it is this is mostly irrelevant.
- Email is protected in-transit anyway, on any well configured mail service. The connection from foo.com to bar.com would be encrypted using TLS, taking it out of the reach of a passive man-in-the-middle adversary anyway.
I'm the operator of nextlinemail. I can share how our service operates, but it may not be true of other services.
Our webmail client is only available over TOR, via a .onion link. When you send email, however, we use a series of rotating relays to deliver the email to the clearnet. This email behaves exactly the same as normal email.
When receiving email, the same is true. Our servers are accessable via the clearnet for other email relays to access. Email is delivered to your @nextlinemail.com email address over clearnet. You then access your email via TOR, through our webmail client.
I believe this is a common pattern with other mail providers such as secmail, tutanota, and protonmail.