Have used it and believe it’s only really secure with others who also use it. When you email others who use other providers there is no ‘encryption cooperation’ in a sense, then. I could be wrong, but nobody else uses it with me and many feel free to pass things I say to them on to others anyway, etc., so all the cautionary measures become useless. In short, if others cooperate by getting their own accounts with proton to communicate with you – yes, then it should “be good.”