I made a thread asking this same question: How are photons emitted at longer wave lengths then the shortest electron hop (some where in IR from a molecular bond I think) I was trying to figure out how photons are emitted from metal antennas and the area between far IR and mm waves. There should be a well defined cut off.
It is difficult to explain in everyday terms, because this area of physics starts to exhibit quantum phenomena that behave differently to everyday phenomena humans understand. Simply put, it is in many ways nonintuitive. Trying to explain it to someone without a suitable basis, is like trying to explain why bumblebees can actually fly to someone who has no idea about aerodynamics, and has no reason to believe what you say.
I won't be able to convince anyone about this either, but I'll try to explain it in terms that let you build a rough picture/understanding of the stuff involved, and if you are inclined to do so, research the details.
The first thing to understand is that metals in some ways behave as a single large molecule. That is, there are
free electrons, or more properly,
delocalized electrons in the lattice formed by the atoms. (Only the outermost electron or electrons of each atom are delocalized or "shared" this way.) In many ways, these electrons can be modelled as if they formed a gas, electron gas or
Fermi gas. Because of the regular lattice structure, there are a lot of states (corresponding to electron orbitals in molecules); so much so, that they're not even individually numbered, but described using a curve,
density of states. The curve describes the number of states as a function of energy; and because the entire lattice of atoms is involved, it is (can be) continuous: the difference in energy between different states can be infinitesimally small.
When there is no external energy input, the electrons are at their base energy states, and they occupy all the lowest energy states. The states that are normally filled are called the valence band, and the states that are normally empty are called the conduction band. (The name "band" comes from them forming a band in the density of states graph. It is not a physical band like in geological strata or a nice strawberry cake.)
If the valence and conduction bands are right next to each other, you have a conductor. If there is a large gap in between, with no electron states at all, you have an insulator. If the gap is small (less than 4 eV), you have a semiconductor.
When a semiconductor material is excited somehow (so it absorbs energy), some of the electrons jump from the valence band to the conduction band. When they relax back, the electrons emit a photon with energy corresponding to the band gap size. There is really no lower limit to the band gap; only when it is very small, the material properties blur between a conductor and semiconductor.
Conducting metals have no such band gap. The valence band and the conduction band are right next to each other in energy. So, there is no lower limit to the photon energy when an electron drops to a lower energy state in the Fermi gas, because the density of states can be continuous: that is, there is no lower limit between the energies of two different electron states.
This is a very simplified picture, mind you. The entire lattice itself can vibrate (acoustic waves,
acoustic phonons), as can the electron "gas" (especially at the material surface). These all interact, for example in
sonoluminescence (we just do not know
exactly how). Even the interactions in something as dull as a chunk of room-temperature iron are quite complex and very, very fascinating: not simple at all. Water, on the other hand, is so complicated to model that it frustrates many a materials physicist/quantum chemist/quantum biologist every day.