Offer integration into GNOME Online Accounts (GOA)
I just started using Tutanota as my default email, and it seems to be quite nice. I love the fact that this software's open-source, so I can easily drop Gmail.
However, I think it'd be interesting/convenient to offer connection with GNOME's Online Accounts system. I noticed in the past that someone did provide feedback for IMAP/POP3 access; however, this could be something new and interesting to look at, especially with GNOME's sandboxing features.
I am unsure if you can bake in the end-to-end encryption, but here's the link to the project page: https://wiki.gnome.org/Projects/GnomeOnlineAccounts