  We explore the electromagnetic gauge invariance of the hadron tensor of the
Drell-Yan process with one transversely polarized hadron. Due to the special
role of the contour gauge for gluon fields, the prescription for the gluonic
pole in the twist 3 correlator can be related to the causality prescriptions
for exclusive hard processes. Because of this, we find the extra contributions,
which naively do not have an imaginary phase. The single spin asymmetry for the
Drell-Yan process is accordingly enhanced by the factor of two.
