Plus their unwillingness to open source it and not sharing the audits just doesn’t inspire my confidence.

The server side isn’t open source, and you can’t verify that is what is actually running in production. While we do recommend it I don’t personally use their products.I like the use of email clients, particularly customized to my needs.

Nested folders was only a very recent feature added tutanota.com/blog/posts/subfolders and without that I wouldn’t even consider a provider as I use this for organization. Of course as you can’t use your own email client, downloading email from Tutanota can be a bit of a pain too, you can only export per-folder into Mbox.