[nbraud]

I'm not sure MX records would work without either modifying SMTP servers or using Tor in transparent proxy mode.

Postfix, for instance, has [transport maps](http://www.postfix.org/transport.5.html), but those let you pick a transport based on the email domain, not the MX host.

Right now, some people use [onionmx](https://github.com/ehloonion/onionmx), which maintains both a [static mapping](https://github.com/ehloonion/onionmx/blob/master/sources/map...) from domains to onion services, and a dynamic one using [SRV records](https://github.com/ehloonion/onionmx/blob/master/SRV.md).