Probably the most well-known open-source email application is Thunderbird. But it's clearly on life support, and at least in my experience, it has never been very robust. Plus, it's an old-school desktop application, and while some of us (myself currently included) prefer that, most people prefer to do email in a web browser and on a mobile device.
There are open-source webmail applications like Roundcube, but at least last time I looked, they required frequent round trips to the server, albeit with some JavaScript enhancement. In modern parlance, they're not offline-first.
So is there already a good open-source, offline-first email application that can run on multiple platforms (i.e. not something like K9Mail for Android)? If not, what might be the best way to go about developing one? I'm guessing the best starting point would be the email application from Gaia [1], part of the now abandoned Firefox OS.
[1]: https://github.com/mozilla-b2g/gaia