The Nextcloud mail app does not support OpenPGP. Many similar webmail platforms do support PGP indirectly though the open source Mailvelope browser extension. The Nextcloud mail app does not work with Mailvelope due to the inability to display the PGP/MIME attachment in the browser.
I have spoken to the Nextcloud company and their funding model does not support commissioned feature development. They also do not see this feature as a priority. Although the ideal solution would be to find a way to introduce the feature into the official Nextcloud app and thus offer the capability to the community, this doesn’t seem possible at this time.
My proposal is to fund a Nextcloud app developer to fork the Nextcloud mail app code into a new app that can display these PGP/MIME attachments in the browser thus giving the Mailvelope extension the ability to scrape the data and work with PGP.
If any Nextcloud app developers have the ability and interest to accomplish this project, let’s talk.