Proton Mail
Swiss end-to-end encrypted email. Proton publishes the official onion mirror on its own website.
Links: services with an official Tor mirror
1 service hand-picked. Each card shows the regular website next to the onion mirror published by the same organization, so you can verify the address at the source before you use it.
Onion links open only in Tor Browser. Always compare an onion address with the one shown on the organization's own clearnet page before logging in.