It confirms you have an x86_64 (64-bit) kernel and you are indeed using the test system. It's not supported on the forum, unfortunately, because some things don't work. But you're right, most of the time it works well. I'm using it now, as it happens.
How knowledgeable are you about Linux? If you would like to become a tester and report on what does and doesn't work and test new packages as they come out, you could sign up as a tester. There's a thread in the announcements section. If you could learn to package and help fix things that need doing, even better.
However, it looks as if your current choices are: install 32-bit where everything should work, or use a different mail client.
You need to be aware there's no guarantee on anything, but especially on 64-bit if your data gets lost on what is still regarded as experimental. You use it even more at your own risk (if that makes sense - I know it doesn't really) than the 32-bit (which, like all Linux, is AYOR).