Astropathic transmissions are<em>Incredibly</em>safe and reliable, since they use coded ciphers which the astropaths themselves don't know.  (a typical arrangement features a cryptology servitor who interprets the coded transmission repeated by the astropath, then hands the parchment directly to the senior crew.  With a memorance implant, the crew can even do this themselves.)<br/><br/>Astropaths are soul bound to the emperor, which makes them highly resistant to heresy and daemonic incursion.  (basically, they'll die instead of be corrupted when exposed to warp energies... most of the time, anyway.)<br/><br/>Now, secure transmissions between Chaos ships are impossible, because they send demons through the warp as couriers, which can be intercepted incredibly easily by learning their true name and summoning them.  Chaos fleets usually maintain a list of truenames, and can get badly disrupted if someone summons and slays all of their demons.  (though using sorcery to interrogate them first could be valuable)<br/><br/>So, in summary, Chaos is really good at infiltration, but the Imperium has the advantage in secure communications.  Astropathic signals only get intercepted if there's a spy directly in or  bugging the astropathic choir chamber (most likely with psychic powers, since electronic devices would make the astropaths' heads ache like crazy)   Usually, the astropathic choir chamber is the second most heavily guarded area of the ship, behind the navigator spire.