  Despite the first detection of X-rays from massive stars occurring four
decades ago, the physical dependence of the emission mechanism(s) on the
underlying stellar and binary properties of the emitters remains uncertain. The
young massive cluster Westerlund 1 provides an ideal testbed for understanding
this phenomenon, with over 50 cluster members detected in historical X-ray
observations. In the decade since these data were obtained, significant new
multi-epoch observations of the cluster have been made, allowing a fundamental
reappraisal of the nature of both X-ray bright and dark stars. A total of 45
X-ray sources within Wd1 now have precise spectral classifications. These
observations identify 16 candidate and confirmed massive binaries; by
comparison 22 stars have X-ray properties that imply a contribution from a wind
collision zone. X-ray emission appears confined to O9-B0.5 supergiants,
Wolf-Rayets and a small group of highly luminous interacting/post-interaction
binaries. Despite their presence in large numbers, no emission is seen from
earlier, less evolved O stars or later, cooler B super-/hypergiants. We suppose
that the lack of X-ray emission from O giants is due to their comparatively low
bolometric luminosities if, as expected, they follow the canonical L_X/L_bol
relation for hot stars. The transition away from X-ray emission for OB
supergiants occurs at the location of the bistability jump; we speculate that
below this limit, stellar wind velocities are insufficient for internal, X-ray
emitting shocks to form. Our results are consistent with recent findings that
massive binaries are not uniformly brighter than single stars of comparable
luminosity and spectral type, although it is noteworthy that the brightest and
hardest stellar X-ray sources within Wd1 are all either confirmed or candidate
massive, interacting/post-interaction binaries (Abridged).
