What message is Edgar Allan Poe sending in his story Masque of the Red Death?
This is an interesting way of phrasing the question. Normally, the unspoken meaning is that we are meant to read and analyze the story and come to a conclusion about what the message of the story is, and assume that this is what Poe intended. However this is actually somewhat presumptuous; for example, it presumes both that the author's message is evident in the text, and that it is the same as the one we interpret, among other things. Basically, this type of question seems to invite us to investigate the author's intentions, but in practice it usually ignores this and focuses instead upon our own interpretation of the available material, and thus it has no real basis in what Poe's message actually is.
What would really help to understand Poe's intention is reading about how he wrote Masque, and not necessarily the story itself. At least part of the inspiration for the story almost certainly comes from the context in which it was written; Poe's wife was unpredictably ill at the time, which may have caused Poe to become unusually pessimistic, as well as to drink excessively.
Masque is often cited as one of Poe's strongly moralistic and thematic works because of its conclusion; " - the Red Death held illimitable dominion over all". In context, this seems to suggest that Prospero and his nobles represented human ego, and the conceit that money or vanity could isolate one from death. This is closely related to similar gothic themes in Frankenstein, where ego plays a role in seducing characters into a false sense of superiority over nature. However, if there is an actual moral to the story, it is not stated; in fact this may simply be an exercise in gothic themes, amounting to little more than a successful pulp story.
An elegant way of summarizing this story's (apparent) message is in an Italian proverb related to chess: "When the chess game is over, the pawns, rooks, knights, bishops, kings, and queens all go back into the same box." Humans, regardless of their station in life, have no power over death.