Say user Bob sends an encrypted message to a server. People can download the message from this server and later get the key directly from Bob.
Is it possible for the server to somehow verify that users are able to decrypt the message later on with Bob's key without letting the server be capable of decrypting the message itself? So the server knows the message is being opened but itself can’t “see” the contents? Is it – at least theoretically – feasible to have an implementation which allows for this situation?