This is a huge success because similar figures have never been reached with PGP - even though the protocol was introduced over 20 years ago. The reason is simple: PGP is rather complicated - cryptography experts like Matthew Green even argue that "It's time for PGP to die".
One of the major problems why PGP has not become mainstream is that the other person needs to use it as well. This is rarely the case. So even if one person makes the move and wants to use PGP, they soon get frustrated because none of their friends wants to make the move as well.
This fact, however, is one of the reasons why so many Tutanota emails are encrypted: You can force your friends to use encryption. By adding a password to your contact, you can send an end-to-end encrypted email to any friend in the world, even if they use a Gmail account. Once they have seen how easy it is, many also sign up and switch to Tutanota.
There is no reason NOT to encrypt anymore
Even to people not very interested in protecting their privacy, there is simply no reason anymore to let the spies look into their mailbox while they can have exactly the same product with automatic encryption.
Maybe this is the biggest success of Tutanota: We don't need to convince you to trade in convenience for security. With Tutanota you'll get both.
Tutanota takes away the hassle by automatizing the encryption process. This makes it so easy that you don't even notice that you are emailing end-to-end encrypted. Everybody can do it, even your mother. This high level of convenience - all you have to do is letting your friends know about your new email address - leads to entire families or groups of friends signing up.
Of course we can't really know this because we - as well as third parties - can't read your emails or your contacts. But people tell us how they made their entire family sign up when they send emails to our support team. The tone of these emails to us makes us smile time and time again: They are so happy that finally they have found a way to communicate securely with family and friends.
We are very happy that the percentage of end-to-end encrypted emails is rising so quickly. This makes our online communication as a whole much more secure as we make it harder for the spies to monitor everything. Mass surveillance is an intrusion on everybody's right to privacy so it needs to stop. If Secret Services deny to change their habits, we have to change ours: