Add support for POSTFIX_OVERRIDE_FROM.

@@ -29,6 +29,13 @@ postconf -e \
 : > /etc/postfix/virtual
 postmap /etc/postfix/virtual
 
+if [[ $POSTFIX_OVERRIDE_FROM ]]; then
+	postconf -e smtp_generic_maps=regexp:/etc/postfix/generic-regexp
+	echo "/^(root|hostuser)(\\+.*)?@[^.]+$/ $POSTFIX_OVERRIDE_FROM" > /etc/postfix/generic-regexp
+else
+	postconf -e smtp_generic_maps=
+fi
+
 : ${POSTFIX_FORWARD_APP:=hostuser+app@localhost}
 : ${POSTFIX_FORWARD_CATCHALL:=hostuser+catchall@localhost}