What are zero-knowledge email systems?

Are we good with this definition for zero-knowledge email systems?

Zero-Knowledge privacy means the server is never capable of viewing plain-text data; therefore, the data may never be compromised through mismanagement, prying eyes, or external bodies looking to gain access.

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I'm making some corrections to the Compare - Encrypted Email Service Providers

I'm also a bit puzzled about some of the zero knowledge claims since it seems that zero knowledge should require encrypted metadata.

@ProtonMail @Tutanota @SCRYPTmail @kangas

popowich said:

I'm also a bit puzzled about some of the zero knowledge claims since it seems that zero knowledge should require encrypted metadata.

Click to expand...

If zero knowledge should require encrypted metadata why does the chart show the half shaded circle for encrypted metadata while the zero knowledge shows a green check.

Even though the word "should" is used,there seems to be some doubt.