That mail client, lttrs, made by the developer of XMMP app Conversations, looks like a proof of concept to me rather than a full fledged email client. I’ve tested it with https://stalw.art mail server, which supports JMAP and it was not a piece of cake to make it all work together. Stalwart is more difficult to install and configure than Yunohost but it is expected to have an admin interface in a few months time. I have the impression that it would be possible to run Yunohost and Stalwart on the same server if you use non default ports for Stalwart.