We study the return probability and its imaginary (τ) time continuation after a quench from a domain wall initial state in the XXZ spin chain, focusing mainly on the region with anisotropy |∆| < 1. We establish exact Fredholm determinant formulas for those, by exploiting a connection to the six vertex model with domain wall boundary conditions. In imaginary time, we find the expected scaling for a partition function of a statistical mechanical model of area proportional to τ 2 , which reflects the fact that the model exhibits the limit shape phenomenon. In real time, we observe that in the region |∆| < 1 the decay for large times t is nowhere continuous as a function of anisotropy: it is either gaussian at root of unity or exponential otherwise. As an aside, we also determine that the front moves as x f (t) = t √ 1 − ∆ 2 , by analytic continuation of known arctic curves in the six vertex model. Exactly at |∆| = 1, we find the return probability decays as e −ζ(3/2) √ t/π t 1/2 O(1). It is argued that this result provides an upper bound on spin transport. In particular, it suggests that transport should be diffusive at the isotropic point for this quench. arXiv:1707.06625v3 [cond-mat.stat-mech]