Naven
Glossary
Metadata
Everything about a message except what it says: who sent it, to whom, when, how big it was, from where.
Metadata is often more revealing than content, and it survives encryption. Knowing that somebody called a suicide hotline for eleven minutes at three in the morning says more than a transcript would. Knowing which accounts talk to which, and how often, reconstructs a social graph without reading a word.
This is why "we cannot read your messages" is an incomplete answer to "what do you know about me". The follow-up question is what is retained, for how long, and who it is given to.
Where Naven stands
Naven stores what it needs to deliver messages and no more, and its privacy page says what that is rather than gesturing at it. There is no advertising business, which removes the usual reason to accumulate more.