After googling and looking the words 'but' and 'what' up in the dictionary and doing some thinking and guessing I came to the conclusion that it essentialy means this (omitting the unnecessary parts of the sentence): Who knows that the message may not be received (some day)? (or 'if the message won't be received')
Compare with a similar sentence (from Random House Kernerman Webster's College Dictionary):
Idioms:
1. but what, Informal. but that: Who knows but what the sun may still shine.
And here's a whole bunch of them in one passage:
Nobody knows to the contrary. I cannot tell but what God may bless you to this entire nation. Nobody will dare to say that he cannot. I cannot tell but what God may bless you, my friend, to that part of London in which you live, even though you may be deeply conscious of its great needs, and of your own insufficiency. Who can tell what the Lord can or will do? Dear mother, who knows but what the Lord Jesus may bless you to all the members of your family, so that by your moans all the little ones shall come to him? Nobody has any right to speak to the contrary. Who knows but what God may bless you, dear teacher, to all your Sunday-school class, so that you may meet them all in heaven? Nobody can declare that it shall not be so, therefore strive after it.
https://www.spurgeon.org/resource-libra ... #flipbook/
The word 'but' itself is sometimes used to mean "that... not", as in:
There never is a tax law presented but someone will oppose it.
No leaders ever existed but they were optimists.