Infallible Messenger is one of my favourite Sorceries; besides allowing for communications over staggeringly vast distances, it opens the door to a variety of hijinks such as pumping up perfect social attacks for remote subversion of enemies, Draft Horse Measuring yourself to the cherub for transportation, tracking the blighter to find people mundanely hidden so well you can't find them, and other wonderful shenanigans. Unfortunately, I just realized I can't figure out if it works on Alchemical Exalts, and I'm in an Alchemicals game. (Yeah, I can't cast it either way, but it's pretty easy by sorcery standards so I assume I'll run into some Creation-born allies with this trick sooner or later.)
The cherub is a spirit, and can "travel from one point to another in Creation or Yu-Shan nearly instantly." The sorcery states that "unless the target's location is mystically obscured, the cherub will find the recipient" and all the sorcerer needs to do is "name and describe a target."
My first doubt is because Alchemical exalts are outside of fate. That messes with a lot of heavenly ways of finding people, which I would assume the spirit is using. (I don't actually know how the cherub finds its target, and have no idea where that might be detailed if it ever is.) Still, the sorcery doesn't mention that as being a problem, even though things exist outside of fate in the book the sorcery is described in, so I suspect it can find people outside of fate. My second doubt is because Alchemicals may very well be in Autochthonia, which isn't Creation or Yu-Shan. (Assume for the purposes of this question that the seals separating Autochthonia from Creation have been broken and Alchemicals roam abroad in Creation.) Obviously Autochthonia isn't Creation or Yu-Shan and the cherub isn't supposed to be able to reach people in Hell, the other plane mentioned in the corebook. This leads to the assumption that it can only access those two planes.
TLDR, in Exalted 2.5, can the sorcery Infallible Messenger be used to contact someone who is outside of fate? Can it be used to contact someone in Autochthonia, assuming that there is a beachhead to Creation that's been established? My reading of the rules is yes and no respectively, but I can't come up with any convincing reasons why this is so.