As an example, if communication was done using visible light, communication would be limited by the respective atmospheres, dust and obstacles along the line of sight. At the same time, multiple channels could propagate in parallel, just as we can image the lunar surface from Earth as a plane of pixels instead of solving only a single dot. So if the entire surface of both planets are used for communication, bandwidth raises, as long as resolution is sufficient. There are many limiting factors in interplanetary communication, and my intuition is that the highest the carrier frequency of one channel (one EM pixel), the more difficult it is to propagate it reliably in such a point to point access. I may be wrong about that, although it is well known cellular networks, reaching higher frequencies do require a tighter grid of antennas. Lastly, I know there is a different answer for every couple of planets one chooses. So here are two examples that cover what I am interested in: Earth to Neptune, and Earth to the closest known planet from a different stellar system (4 light years, Proxima Centauri b or c). My guess is we cannot achieve reliable communication with planets far away, just a few bits per second top, since we almost cannot even image exoplanets. My question is about the physical limits of such communication, not the technical barrier.
Edit: communication only through electromagnetic radiation.