In your example, unless your send_mail procedure is generic over transport in some way, surely its use of TCP is an inherent part of its behaviour, not an imple

In your example, unless your send_mail procedure is generic over transport in some way, surely its use of TCP is an inherent part of its behaviour, not an implementation detail? You're going to have to handle them as TCP errors anyway, even if they're lifted to a different type, so why not just throw them as they are?