content: Via the web client or the android/iOS apps.    I too am a little bummed with no support for third party mail apps but I can recognize the issue.  Protonmail is not just a email service (those are a dime a dozen).  What they are is a secure email service and guaranteeing security with a POP3 connection is practically impossible as you cannot guarantee the security of the e-mail once the client has copied it. You and I may not care and understand the risks of local storage but the average user probably doesn't. So Protonmail is trying to keep it simple and don't support the option. That said, they have claimed to be working on a IMAP solution.  In theory IMAP mail doesn't need a local copy buy in practice many apps will store a local cache of mails. Not sure how they handle that issue but from a post they made at the beginning of the year it sounds like their IMAP solution involves placing a self contained local IMAP server on your machine that handles the email storage (and communication with the protonmail server).   It was suppose to be available mid 2017 but I haven't heard an update about it recently (supposedly they had something in Beta in the spring).  Hopefully they come up with something but it sounds like it will only work on "real" computers that can run the local IMAP server.  It likely won't support third party clients on tablets or phones. Just a guess though.