My understanding is that such messages go to other Astropaths. Surely when using astrotelepathy you know who you are sending to. If you don't know that, how do you even know they have their own Astropath to receive the message?
Much of the time, I surmise the sender of an astropathic signal chooses the place, and hopes for a receiver. With such vast distances, and time being so wanky thanks to the Earp, you have no idea where anyone ever is. Say you wanted to reach the Inquisitor Council on Scintilla. They have astropaths, but any particular one might have had an incident, and blown themselves up, been killed through the machinations of others, been called away for another reason, or whatever. You really don't have a good way of knowing which Astropaths are in the Tricorn, at the time, and who will still be, when your message finally gets there. I believe you send to a location, skipping it off the heads of other Astropaths, as it goes, to maintain integrity, and it eventually gets to Scintilla, to the Tricorn, and to an Astropath of the Council, who decodes it, and takes it to the Council, or the Lord Sector, or whoever needed to know what's up.
Something I'm sometimes confused by, when the Horus Heresy happened, half of almost everything fell to the Ruinous Powers. They didn't JUST get way too many Space Marines, they got what became the dark Mechanicus, whole chunks of the Imperial Army, and many of the ships and vehicles that came with them. Since then, numerous other individuals have also succumbed to the taint of Chaos. How does Chaos have no "astropaths" on their side, intercepting the messages, who know the same ciphers? I'm assuming it's the Soul-Binding, in a more RP version than the as written mechanic, but I don't know, and with everything else Chaos also has, it's kind of odd that the one thing they didn't pick up was Astropaths, to listen in on the Imperium's messages.