Imagine a photon stretched in space along the direction of its motion from end to end of photon's world in range equal to the age of universe * c. In this way, each photon of background radiation is stretched, and such a photon passes through every point of space from each side. And what will happen to these photons, when the universe in their direction of motion, according to Hubble's law, stretches more than it is allowed by the limit of speed in Doppler formula, which must confirm their redshift? It's going to be the square root of the negative. There is no redshift of light, that cannot reach us because of the expansion speed v>c, but the velocity in Doppler equation that gives the redshift of CMB photons that are everywhere, this velocity doesn't know it and it's going to exceed c, resulting in a square root of negative and previously zero in the denominator for v=c.
If u care, read also my answer starting with "The speed we are talking about is the space expansion rate itself".