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I think their servers.

(This is my imagination and it might be completely wrong so please feel free to correct me)

they mentioned spam problem which leads me to believe that if enabled, the sending client will encrypt the whole message using the recipient's public key and put all metadata other than recipient's identifier inside this bigger encrypted envelope. The receiver opens this envelope with its key and opens the smaller box inside which contains the sender's metadata.

What we lose now with this is the server does not have much insight into who is sending messages (by design). This means if you allow sealed sender from everyone, someone could send you a lot of messages which you may not like.