Daniel Aleksandersen comparing these:

Both of these email service providers focus on customer privacy. Both offers webmail front ends with support for Pretty Good Privacy (PGP) end-to-end encryption in addition to PGP for at-rest encryption of all messages directly from the web browser.

So how do these two services with similar goals compare head to head?

ProtonMail is better designed, but more expensive and more proprietary. Daniel prefers Mailbox.org because of the open standards and data portability.