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The observed Maxwellian velocity distribution functions in plasmas and the fact that the rate of elastic electron-electron is many orders of magnitude smaller than the electron plasma frequency has been a long-standing puzzle. Here, we present a mechanism for the efficient thermalization in collision-poor unmagnetized plasmas that resolves this puzzle. The competition between the momentum losses of plasma particles by spontaneously emitting high-frequency non-collective fluctuations and the momentum diffusion of these particles in their self-generated fluctuating electric field fluctuations provides the Maxwellian particle distribution function. The mechanism is self-regulating, providing electron temperatures of about 107 K, and is applicable to fully-ionized plasmas with electron densities below 1027 cm-3.