-with some conditions under which it is done, it can be selected here. */
-
-if (ob->widen_domains != NULL)
+with some conditions under which it is done, it can be selected here.
+
+The rewrite_headers option works only when routing an address at transport
+time, because the alterations to the headers are not persistent so must be
+worked out immediately before they are used. Sender addresses are routed for
+verification purposes, but never at transport time, so any header changes that
+you might expect as a result of sender domain widening do not occur. Therefore
+we do not perform widening when verifying sender addresses; however, widening
+sender addresses is OK if we do not have to rewrite the headers. The
+suppression of widening for sender addresses is silent because it is the normal
+desirable behaviour. */
+
+if (ob->widen_domains != NULL && (verify != v_sender || !ob->rewrite_headers))