Introducing ProtonMail's Tor hidden service
This is not quite as good as riseup.net's onion support as it doesn't include SMTP services. See:https://riseup.net/en/security/network-security/tor#riseups-...  mike@snake:~$ torsocks telnet wy6zk3pmcwiyhiao.onion 25
  Trying 127.42.42.0…
  Connected to wy6zk3pmcwiyhiao.onion.
  Escape character is ‘^]’.
  220 mx1.riseup.net ESMTP (spam is not appreciated)

So if your mail service supports onion addresses, then you can just replace "@riseup.net" in a users email address with "@wy6zk3pmcwiyhiao.onion".Alternatively, your mail service could have explicit configuration in place to identify @riseup.net addresses and route them to wy6zk3pmcwiyhiao.onion instead of the normal MX records. I do this with Exim by utilising Tors TransPort+DNSPort functionality and then adding the following Exim router:  riseup:
    driver = manualroute
    domains = riseup.net
    transport = remote_smtp
    route_data = ${lookup dnsdb{a=wy6zk3pmcwiyhiao.onion}}

Obviously this would be better if there was a way to dynamically advertise the onion address in the DNS instead of having to hardcode it in Exim.[edit] - If they co-ordinated, Riseup and Protonmail, and potentially other similar privacy respecting mail services could send all their traffic over each other via Tor. If you work for either of these companies, please consider the possibility of looking into this sort of relationship.