We have a customer using Daylite with Apple Mail on the macOS. The guys from marketcircle have ahd a very good idea on how to integrate Mail to their software. To me their approach makes a lot of sense as most people or not willing to give away their Thunderbird, Outlook etc.
Here is a quick description of how it works:
https://www.marketcircle.com/help/article/how-can-i-add-an-email-into-daylite-using-dma/
We would be very much interested on helping to integrate such a plugin for Thunderbird.