  We study the properties of X-ray emitting plasma of MP Mus, an old classical
T Tauri star. We aim at checking whether an accretion process produces the
observed X-ray emission and at deriving the accretion parameters and the
characteristics of the shock-heated plasma. We compare the properties of MP Mus
with those of younger classical T Tauri stars to test whether age is related to
the properties of the X-ray emission plasma. XMM-Newton X-ray spectra allows us
to measure plasma temperatures, abundances, and electron density. In particular
the density of cool plasma probes whether X-ray emission is produced by plasma
heated in the accretion process. X-ray emission from MP Mus originates from
high density cool plasma but a hot flaring component is also present,
suggesting that both coronal magnetic activity and accretion contribute to the
observed X-ray emission. We find a Ne/O ratio similar to that observed in the
much younger classical T Tauri star BP Tau. From the soft part of the X-ray
emission, mostly produced by plasma heated in the accretion shock, we derive a
mass accretion rate of 5x10^{-11} M_{sun} yr^{-1}.
