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Maybe I'm confused, do the DeltaChat and ArcaneChat clients only work with DeltaChat/ArcaneChat servers?

The "ArcaneChat/DeltaChat servers" are just normal email servers with some default configurations and tweaks for privacy/security and speed

Edit: forgot to mention I can see the sender & recipient addresses (Signal uses sealed sender to minimize this metadata leak)

Signal needs to "seal sender" to be able to send messages anonymously since their service is not anonymous and you login with your phone number, in ArcaneChat it is like you are "sealed sender" from the very beginning, you don't register with phone number or any private data, you log in anonymously always, currently you have an static anonymous identity, and have to manually change it over time if you are the most paranoid person in town, but in the future the app might implement anonymous identity rotation

I can also see what time the message was sent this is the kind of metadata Meta collects through Whatsapp even though they also encrypt message content.

Nothing that the server doesn't know, the server knows the time at which you try to send a message because well you are asking it to do so at that time. But I agree this is a problem with stored messages if the server gets audited at a later point, by default with a single device messages are deleted immediately and otherwise after 20 days so still it is limited what they could get, but this can be improved, the header doesn't need to have a real date could be whatever fixed date while the real date is protected in the encrypted part, this needs to be done 👍

It doesn't seem - although maybe it now does - that DeltaChat nor ArcaneChat support key ratcheting, so if someone's intercepting messages they can decrypt all future + past messages.

This is a pretty theoretical situation, first the attacker needs to get control of your chatmail provider/server and start collecting your messages, secondly you need to happen to be using disappearing messages since otherwise when they get access to your phone to get the key they can as well just get all your messages that are available already decrypted in the app, since you need the messages to be ephemeral, in that case you can as well create a temporary profile, ex. For some protest or activism and delete it after the operation is finished, and you get the same results of "forward secrecy" without sacrificing the usability of the app, ex. In ArcaneChat it is possible to have your account in as many devices as you want all well synchronized and every device is totally independent, if your phone dies you can keep using it in other devices or add it back to a new phone without losing a single message