What does Poe mean by "so far every thing is within the limits of the accountable." when speaking about the poem The Raven
What Poe means by this is that everything in the poem is "possible" - none of it defies reason. A sound at a window, a tapping, a bird coming through the window, and even the fact that the raven is able to speak the word "nevermore" is within the realm of possibility. Birds can imitate human speech, and the word "nevermore" is not a difficult word to mimic phonetically. As Poe notes:
Here, then, immediately arose the idea of a non-reasoning creature capable of speech, and very naturally, a parrot, in the first instance, suggested itself, but was superseded forthwith by a Raven as equally capable of speech, and infinitely more in keeping with the intended tone.
One of the things that Poe is discussion in his philosophy of composition is just that - the realms of possibility in fiction and those elements that stretch possibility but not that defy possibility.