AFAIK, Mailbox.org was once open source, but has not very recently become closed source. PrivacyGuides.org recommends it, as does Tresorit (encrypted cloud storage), for example, which is closed source and is one of the services I use. Providers that implement encryption, have been audited, have been on the market for years, and have a clean track record of security or privacy scandals, which there aren’t many of these types of services these days. But I personally don’t like them and try my best to avoid closed source softwares as well.

In conclusion, PrivacyGuides mostly recommends open source software but also recommends some that are not based on their features, reputation, security and maturity.

Mailbox.org is closed source unfortunately and I believe the Client is too

https://www.reddit.com/r/ProtonMail/comments/1i2ff6q/call_for_andy_yen_to_resign/

UPDATE: Andy Reply

According to Andy’s logic, if Hitler were the president of some unfortunate country, we should differentiate the boss from his good nominees. Even using a company founded by an entire community to show a good evaluation made by one of its founders to give him a loving pat on the back and show the world that he is not completely bad as they think, but not meaning that the founder agrees with all his innocent actions, of course, such as disregarding the rights of many people around the world because they are just part of the democratic game.