Two primary criteria are the ability to use two or more different e-mail accounts, and to implement OpenPGP standards[#3156 and #4880]. This is not actually that difficult to find – quite a number of webmail products have excellent implementations of the standard. But adding a requirement of being simple to install and maintain and suddenly the field of competition seems empty.
Splitting the two primary criteria, though, and suddenly the options fill back in.
Using Mailvelope for Chrom(e|ium) or Firefox one can use nearly any webmail interface to send and receive PGP-encrypted e-mails. This project was originally developed in Germany where it is now heavily integrated with several large webmail services. It is also detected and supported by Roundcube, one of the top open-source webmail applications. It is criticised as not “end-to-end encrypted” by some.
One unfortunate circumstance is that the Mailvelope extension for Firefox does not work out-of-the-box with the Tor Browser Bundle for Linux.
I was pointed to Mailvelope, though, while testing out the Mail app for Nextcloud. Mail works reasonably well for a webmail tool. It is not attempting to recreate the wheel – it is pretty much the Horde under the hood. What it is doing, however, is integrating with Nextcloud’s sync’ed Calendar, Contacts, and Files. Want to e-mail a file from your cloud? Accept an invite from an e-mail and have it show up on your phone? Search your contacts for so-and-so’s e-mail address and immediately send them a note? yah, that kind of integration.
This integration with our existing family Cloud immediately pushes Mail app to the top of my list of possible tools. A few days of testing will give me a better idea of how well it can work.