Make the support (and website) better
This is a major reason to chose a program or an Internet service over another one. I understand there are priorities, but this part of Tutanota looks too much as if it were still in beta.
You need a much better online manual, FAQ, tutorials, and you absolutely need a dedicated, official forum on your own website.
You also need a better way for one-on-one contact with users, if needs arises. The current email addresses are semi-secret, they are not good enough.
It's quite acceptable to provide this only to paying customers (although doing it for everybody would obviously be better), but it needs to be displayed for everybody, if only to advertise that the possibility is there provided you go premium.
People keep telling me Protonmail is the way to go, and I keep telling them that Tutanota is better in several, major respects, but landing on your website, and landing on Protonmail's, one gets the feeling the latter is in a different league.
You also have several quality German competitors whose website is way above yours.
Committing to an email service is a major decision. Trust is paramount. Especially if you ditch Google (or anything else) for an encrypted provider.
I have selected my current pdf manager (a decision which has far less implications) in part because they have a wealth of technical information on their website, and a terrific forum with helpful official moderators.
Your guys are very helpful when one can get hold of them, but their helpfulness is not yet channeled in an efficient way through your website.
Thanks for your honest feedback! We are already working on a new website and it is nearly finished. It should be released within the next couple of months.