  We analyze how disorder affects the transition temperature $T_{c}$ of the
$s^{+-}$superconducting state in the iron pnictides. The conventional wisdom is
that $T_{c}$ should rapidly decrease with increasing inter-band non-magnetic
impurity scattering, but we show that this behavior holds only in the overdoped
region of the phase diagram. In the underdoped regime, where superconductivity
emerges from a pre-existing magnetic state, disorder gives rise to two
competing effects: breaking of the Cooper pairs, which tends to reduce $T_{c}$,
and suppression of the itinerant magnetic order, which tends to bring $T_{c}$
up. We show that for a wide range of parameters the second effect wins, leading
to an increase of $T_{c}$ with disorder in the coexistence state. Our results
explain several recent experimental findings and provide another evidence for
$s^{+-}$-pairing in the iron pnictides.
