It doesn't matter as the messages are end to end encrypted and the way it is done is continously verified by multiple leading/up-and-coming cryptographers as far as I understand.
This is the huge advantage that Signal has over mail, the default mode in Telegram and pretty much anything there is: it does matter if NSA, FSB, MI5, Mossad, Google and Facebook all have root on a server that all the traffic passes through. To the best our knowledge - long as they don't compromise one of the endpoints - the only thing they'll get is metadata and the only thing they can do is disrupting the service.
This is the huge advantage that Signal has over mail, the default mode in Telegram and pretty much anything there is: it does matter if NSA, FSB, MI5, Mossad, Google and Facebook all have root on a server that all the traffic passes through. To the best our knowledge - long as they don't compromise one of the endpoints - the only thing they'll get is metadata and the only thing they can do is disrupting the service.