The planes of man are the multiple planes of existence upon which mankind (mankind here includes elvenkind, dwarvenkind, and other sentient races) resides. In principle there are an infinite number of planes of man, but only those most recently divergent from our own are accessible. Thus only the plane of Astlan, and alternate versions of Astlan are accessible. Rumors persists of methods of traveling to these planes with alternate physical laws, but the Astral Plane certainly does not link the Astlanian worlds to them. Incidentally, note that not all the divergent Astlanian worlds use the same name. We use the name of our own reality to describe all those planes of man where the same laws of magical science and nature apply. Thus only the planes of man where the same laws of nature and magic apply, are accessible via the Astral Plane. Certainly, no individuals from other planes have men have ever been seen, nor has anyone ever found a point of egress to one of these planes on the Astral Plane
Theories do abound that methods similar to the very direct Demonic Gateway spell could potentially be used to contact such worlds. However, no such spells currently exist.
Finally, there seems to be some sort of minimal quanta of difference necessary to access the true "alternate realities." Theory suggests that one should be able to reach worlds in which a loved one did not die, etc. However, to date, no one has been able to fine tune or adjust any spell tight enough to select out such worlds. Currently only Astlanian-like worlds separated by a certain minimal amount of change are accessible. Typically these are worlds in which major events had very different outcomes.
The reasons for this are not known, however theoreticians have hypothesized a set of Laws of Plausibility that require understanding of extremely advanced mathematics and wizardry to comprehend. Essentially, many of the alternate possibility worlds (i.e. did I have coffee or tea this morning) diverge only for short durations before collapsing back into the primary reality wave function. Hence, some suggest, the reason you may not be able to remember what you had this morning for breakfast in 20 years is because that information has been lost in the collapse of the current probability wave. In simpler, more philosophical terms, neither the past nor the future exist, only the present is real.