I had assumed their reasoning for not taking that approach might be related to metadata at rest, but it seems they don’t use “zero access” encryption for metadata even at rest so I have no idea what technical justification they could have for not supporting IMAP with PGP handled by the email client. The fact that they restrict bridge access to paying subscribers only doesn’t help them avoid lock-in impressions either.

I’m not sure if this is required. Any decent e-mail server uses TLS to communicate these days, so everything in transit is already encrypted.

In transit, yes, but not end-to-end.

One feature that Proton advertises: when you send an email from one Proton mail account to another Proton address, the message is automatically encrypted such that (assuming you trust their client-side code for webmail/bridge) Proton’s servers never have access to the message contents for even a moment. When incoming mail hits Proton’s SMTP server, Proton technically could (but claims not to) log the unencrypted message contents before encrypting it with the recipient’s public key and storing it. That undermines Proton’s promise of Proton not having access to your emails. If both parties involved in an email conversation agree to use PGP encryption then they could avoid that risk, and no mail server on either end would have access to anything more than metadata and the initial exchange of public keys, but most humans won’t bother doing that key exchange and almost no automated mailers would.

Some standard way of automatically asking a mail server “Does [email protected] have a PGP public key?” would help on this front as long as the server doesn’t reject senders who ignore this feature and send SMTP/TLS as normal without PGP. This still requires trusting that the server doesn’t give an incorrect public key but any suspicious behavior on this front would be very noticeable in a way that server-side logging would not be. Users who deem that unacceptable can still use a separate set of PGP keys.

They say the reason for needing their bridge is the encryption at rest, but I feel like the better way to handle wanting to push email privacy forward would be to publish (or better yet coordinate with other groups on drafting) a public standard that both clients and competing email servers could adopt for an email syncing protocol for that sort of zero-access encryption that requires users give their client a key file. A bridge would be easier to swallow as a fallback option until there’s wider client support rather than as the only way.

A similar standard for server-to-server communication, like for automatic pgp key negotiation, would be nice too.

Still, Proton has a easy to access data export that doesn’t require a bridge client or subscription or anything. I think that’s required by GDPR. It’s manual enough to not be an effective way to keep up-to-date backups in case you ever abruptly lose access but it’s good enough to handle wanting to migrate to another provider.

Compared to simplelogin (or proton pass aliases, addy, firefox relay, etc), one other downside of a catchall is in associations across accounts. Registering with a @passmail.net address implies that I use Proton; registering with [email protected] implies I have access to that domain. If 10 data breach leaks have exactly one account matching the latter pattern then that’s a strong sign the domain isn’t shared. If one breached site has my mailing address, my real identity can be tied to all the others.