  We prove that optimal traffic plans for the mailing problem in $\mathbb{R}^d$
are stable with respect to variations of the given coupling, above the critical
exponent $\alpha=1-1/d$, thus solving an open problem stated in the book
"Optimal transportation networks", by Bernot, Caselles and Morel. We apply our
novel result to study some regularity properties of the minimizers of the
mailing problem. In particular, we show that only finitely many connected
components of an optimal traffic plan meet together at any branching point.
