FYI, though this suggestion was rejected (for the reasons commenters highlighted), we will be having a change going in with the next code push, where you'll be able to set a message that people see when they PM you. (Kind of like an email autoresponder thing: it came out of this suggestion.) So, you won't be able to tell if others haven't chosen to receive email notifications, but if you don't, and you want to tell people that, you'll be able to set the message!