Their own encryption technology is kind of useless because impractical. However I don't want to judge whether it's secure or not. You can only send encrypted messages to other tuta users. No pgp / smime support. If you want to send someone without tuta an encrypted message, they will send them an email with a link where they can enter a password you have agreed on. So if you want to communicate with officials, banks, companies etc. you will always disable it. Also those encrypted messages will never be deleted as one might be able to access it via the link later. This causes your 'free' storage to fill up with old garbo.