Abell 3667 is the archetype of a merging cluster with radio relics .
The NW radio relic is the brightest cluster relic or halo known , and is believed to be due to a strong merger shock .
We have observed the NW relic for \sim 40 ksec of net XMM time .
We observe a global decline of temperature across the relic from 6 to 1 keV , similar to the Suzaku results .
Our new observations reveal a sharp change of both temperature and surface brightness near the position of the relic .
The increased X-ray emission on the relic can be equivalently well described by either a thermal or nonthermal spectral model .
The parameters of the thermal model are consistent with a Mach number { \cal { M } } \sim 2 shock and a shock speed of \sim 1200 km s ^ { -1 } .
The energy content of the relativistic particles in the radio relic can be explained if they are ( re ) -accelerated by the shock with an efficiency of \sim 0.2 % ..
Comparing the limit on the inverse Compton X-ray emission with the measured radio synchrotron emission , we set a lower limit to the magnetic field in the relic of 3 \mu G .
If the emission from the relic is non-thermal , this lower limit is in fact the required magnetic field .