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We propose XMM-Newton observations of 3 sub-millimeter bright galaxies openParSMGsclosePar spanning z=2.1-3.6. These SMGs are gravitationally lensed by massive foreground galaxies comma and each has an apparent luminosity >e14 Lsun comma showing no sign for AGN in extensive multi-wavelength analysis. The combination of the sensitivity of XMM- Newton and the lensing magnification comma as well as the intrinsic brightness of the SMGs comma will enable us to study them individually. We will measure the collective X- ray emission from X-ray binaries and hence its ratio to the star formation rate for the first time and to potentially detect extremely obscured AGN over the rest-frame 2-35 keV range. This study will represent a key step in our multi- wavelength campaign to understand the working of such extreme galaxies.