I use org-agenda, the Emacs task manager, as a TODO list and the problem of getting email to work from an onion address has been a remaining very low priority task for the last couple of years. Not many people need this sort of functionality, but as time passes the problems with conventional email get more acute, especially if you are running your own server.
The problems with existing email can be summarized as:
Using onion addresses gets around the above issues. With onion addresses the public key crypto comes for free, so PGP isn't strictly required and the nay-sayers can stop whining. If you're paranoid enough then you can still use it as an extra encryption layer. Using onion addresses also ensures end-to-end security between email servers.
So long as you're willing to put up with a random-looking email address, and your friends are sufficiently geeky, then onion addresses solve a lot of niggly problems.
Recently I've put some effort into making this work on Freedombone and managed to arrive at a solution where you can send email between onion addresses or between an onion address and a clearnet address. Configuring Exim to do this is mind-bendingly tricky, but possible. This is also a sufficiently niche thing that there is not much information available out there, and what information exists is usually either far out of date or just wrong.
Support for onion email addresses will work "out of the box" with a new Freedombone install. There is also an app called bdsmail, which does something similar but using I2P as the transport mechanism. So you can take your pick, whether you're a Tor fan or an I2P fan.