Sure there is; doves and pigeons!
Although there's a chance nature might still loose your message
by actions of hawks and eagles.. although you could try to solve that by
sending an armed drone as it's escort..
I guess relativity indicates 100% is impossible..
There's just no (hu)man-made product that functions in a way that it would
exclude influences of other humans.. no human can build a vault that can not
be cracked or hacked by other humans.. although sometimes it might take a few
hundred years.. and then, there are ways to put it in such a process it would
disintegrate/destroy/delete itself when opened the wrong way..
Maybe only send out message's you'd like everybody to read.. that would be a
way.. no, it's still not safeguarded the intended way, but at least now
you'll be happy when your message gets read by someone else than the intended
receiver, maybe even address the possibility by writing something in it like:
Dear unintended reader, I am still so f-ing curious to, I get that you'd like
to read message's not addressed to you; it wouldn't be the first time someone
grew aware of a frame or plot against them by opening and reading a letter
not meant for them, but, there is no such conspiracy to you from my end! So!
When you're finished reading and have confirmed to your self enough, there is
no such thing in this letter, or my life for that matter, please be so kind
as to send this message through to the intended receiver!? Thank you dearly!
Haha, for real though: I hope ya had a good laugh!
I did when I read the question.. don't take that as making fun of/at the
human posting the original post, in all honesty I really do think it is a
legitimate question to ask..!!! It's just the 'unintended set-up' such a
questions posses to whatever answers follow on it, that's where the humor
takes over and I have a good laugh following because of it, not at it! So
yeah; thanks for that!
Yo!