From your description it sounds like the feature you might be thinking of as walled-garden-ing is end-to-end encrypted (e2ee) emails, which they call “confidential”. The idea is that you can encrypt a message and send it to someone. The message they receive is actually just a link to a publicly-accessible page that Tuta hosts. You give the other person a password that they can enter on that page to read the email you sent and respond to it. If your recipient is also using Tuta, though, when you send an encrypted email it just shows up in their inbox like a regular email.
This is the standard way to handle secure emails, and it’s actually a limitation of the email protocol. The way you would send an encrypted message to someone on another email server is to encrypt the email with your recipient’s public key. Then the message goes to their email inbox like a regular email and they can use their private key to decrypt it (which is what Tuta does if you’re sending an encrypted email to another Tuta user–they already have the recipient’s public key). Email servers don’t have a standard way to send each other public keys for accounts, so if you want to encrypt an email you either have to get the recipient’s public key yourself and tell your email software to encrypt the message with it, or have your provider send a password protected link.
I actually just switched to Tuta. You can still get and receive normal unencrypted emails. The encryption is optional and not enabled by default. I don’t have strong feelings one way or the other yet on the service as a whole. They just added the ability to import emails exported from another service, which is usually something email providers do pretty early on. Currently it’s only available at the $8/month tier, but it’s speculated that they’ll roll it out to the $3/month tier once it’s stable. That’ll be a non-starter for a lot of people. The client UI is simple but functional. It was easy to set up my domain so I don’t have to go into each account and update my email address. Yeah, no complaints so far, but also nothing that blows me away. There’s a free tier if you wanted to just poke around.
From your description it sounds like the feature you might be thinking of as walled-garden-ing is end-to-end encrypted (e2ee) emails, which they call “confidential”. The idea is that you can encrypt a message and send it to someone. The message they receive is actually just a link to a publicly-accessible page that Tuta hosts. You give the other person a password that they can enter on that page to read the email you sent and respond to it. If your recipient is also using Tuta, though, when you send an encrypted email it just shows up in their inbox like a regular email.
This is the standard way to handle secure emails, and it’s actually a limitation of the email protocol. The way you would send an encrypted message to someone on another email server is to encrypt the email with your recipient’s public key. Then the message goes to their email inbox like a regular email and they can use their private key to decrypt it (which is what Tuta does if you’re sending an encrypted email to another Tuta user–they already have the recipient’s public key). Email servers don’t have a standard way to send each other public keys for accounts, so if you want to encrypt an email you either have to get the recipient’s public key yourself and tell your email software to encrypt the message with it, or have your provider send a password protected link.
I actually just switched to Tuta. You can still get and receive normal unencrypted emails. The encryption is optional and not enabled by default. I don’t have strong feelings one way or the other yet on the service as a whole. They just added the ability to import emails exported from another service, which is usually something email providers do pretty early on. Currently it’s only available at the $8/month tier, but it’s speculated that they’ll roll it out to the $3/month tier once it’s stable. That’ll be a non-starter for a lot of people. The client UI is simple but functional. It was easy to set up my domain so I don’t have to go into each account and update my email address. Yeah, no complaints so far, but also nothing that blows me away. There’s a free tier if you wanted to just poke around.