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.