Support DNS-MX resource records with .onion-filtering for TOR as secure/anonymous email transport protocoll
while a lot of bright minds are working on transport and end-to-end content encryption of email, the problem of transport meta-data anonymization is still unsolved.
This can be solved by a network of private SMTP-servers interconnected via TOR hidden-services like Own-Mailbox.
The easiest way to connect the .onion-hostname of a SMTP-server with a clearnet mail-domain is to use the .onion-hostname of a SMTP-server as a primary MX DNS resource record. To avoid leaking by a fallback to the clearnet mail-servers (secondary MX records) it is very helpful if TOR is able to resolve MX-records AND remove non-.onion-domains from the MX-RRs.
Bottom-line: This would allow encrypted and anonymous email communication with TOR onion-routing as transport protocol instead of plain TCP.