  GRB 221009A was the Brightest gamma-ray burst Of All Time (BOAT), surpassing
in prompt brightness all GRBs discovered in ~50 yr and in afterglow brightness
in ~20 yr. We observed the BOAT with XMM-Newton 2.3 d after the prompt. The
X-ray afterglow was still very bright and we collected the largest number of
photons with the Reflection Grating Spectrometers (RGS) on a GRB. We searched
the RGS data for narrow emission or absorption features. We did not detect any
bright line feature. A candidate narrow feature is identified at a (rest-frame)
energy of 1.455+0.006-0.014 keV, consistent with an Mg XII K{\alpha} emission
line, slightly redshifted (0.012) with respect to the host galaxy. We assessed
a marginal statistical significance of 3.0sigma for this faint feature based on
conservative Monte Carlo simulations, which requires caution for any physical
interpretation. If this line feature would be for real, we propose that it
might originate from the reflection in the innermost regions of the infalling
funnel from low-level late-time activity emission of the central engine.
