Most “privacy” webmail providers, such as ProtonMail, Tutanota, and mainstream services like Gmail, Yahoo Mail, Outlook, and AOL Mail, are labeled as proprietary because they use closed-source software, meaning the code isn’t publicly available for review. These services operate on controlled servers, with features and infrastructures managed entirely by the company. The proprietary nature allows them to implement unique features and monetization strategies, but it also requires users to trust the company’s privacy and security practices without independent verification. This contrasts with open-source alternatives where users can inspect and verify the code themselves.

Posteo is in fact open source.

@Charger8232 Lists from #Alternativeto (which this is based on) are usually not very reliable and show information that just isn’t correct on a service or often includes services that are not real Alternatives to what you were looking for in the first place. The only thing that helps against that is doing your own research.

To pick out one example (I don’t have time to them all): they list #Posteo as being proprietary which just isn’t correct. That company is as #opensource as they come.