And Delta Chat encrypts not only messages but also metadata, right?
Although only when used properly - again, there's no point in relying on Delta Chat if your correspondent is going to forward your messages to people on gmail.
well, at least the issue says, they can be removed from the easily visible header, so they could also be encrypted.
but smtp will know anyway.
i think, this is a general issue with "who is messaging whom" - also signal, whatsapp, whatever have to deliver the message and cannot know this afaik when these delivery data are end-to-end-encrypted. so this is not only an email problem.
but with email, at least, there is no "global" server that sees everyhing (no, also not gmail outside us :) and can draw a huge social graph.
Although only when used properly - again, there's no point in relying on Delta Chat if your correspondent is going to forward your messages to people on gmail.