Oh, also, they don't really implement e2e email encryption, which is kinda ironic. Your emails are stored encrypted, and e2e encrypted when sent to other tuta users, but to everyone else, the only way to send an e2e encrypted message is a normal email with a password protected link.
Tuta argues that their system is more secure/private, since pgp doesn't encrypt metadata. I'd argue that not supporting pgp means you aren't serious about encryption. It might be in theory less private, but it's the one encryption standard people actually use for email, and not supporting it excludes their user from sending e2e encrypted mail in a sensible way, and from receiving it at all, unless all their contacts also use tuta. Which might be the point.