K-9 Mail, as its name suggests, is an email application that has been out for ages. Its source code was first published to its git repository back in October of 2008 and since then we've seen a number of forks created from it. There was a Material Design version released for Android 5.0+ thanks to the work from XDA Senior Member Gaukler_Faun, but it has since been discontinued. A widget was released for it last year thanks to the work from XDA Junior Member kbs1 and new works continue to be created thanks to its open source origins.
Wear OS has evolved from a point where applications were only allowed to run on the smartphone itself to having dedicated applications created for it. So XDA Recognized Developer moneytoo wanted to bring the K-9 Mail client to Google's wearable platform as a standalone application. The developer says the current version is still "rough around the edges" with some features crashing, not working, or simply being difficult to reach. But they want to remind the community that this is only the first version and they thought it was good enough to release as they were able to read and send emails with it (as it was tested with Gmail on Huawei Watch 2).