Can you describe what part? If it’s the lack of IMAP it’s because ProtonMail does not act as a typical mailbox. There’s a nonzero chance you can lose all of your email (you need to lose access to your account and access to all of your devices at the same time). Your email is only “readable” at the time of transmission (ie when it’s actively being sent or received). Your received emails (and copies of your sent email) are then one-way encrypted by Proton in your inbox. Your private key that can decrypt them is stored on Proton’s servers, but that key is encrypted with your password and that password is only stored via one-way hash on their servers (you can see where a loss of access now becomes slightly possible). When your Proton client accesses the mailbox, it receives the content and decrypts it locally. Proton has absolutely zero ways of decrypting your email on their own. And their SMTP server does not save a copy of your emails in transit. This is why you’ll see “zero access encryption data at rest” used in reference to Proton. Data at rest is basically data that is stored on an effectively "permanent"medium (ie not RAM, and there are caveats, but they’re edge cases). So when your email is just sitting there (at rest), no one but you can read them. Proton can’t even be forced legally to hand them over because they couldn’t do so even if they wanted to. It’s virtually the most secure email can be out of the box, aside from key management which is still really secure (them having even an encrypted version of your key makes it slightly more vulnerable). If you setup email encryption (ie, something like PGP or GPG), you can make it even more secure, but that has all the same caveats as it does elsewhere. The recipient needs your public key and you need their public key.