permits this.
-.option multi_domain smtp boolean true
+.option multi_domain smtp boolean&!! true
.vindex "&$domain$&"
When this option is set, the &(smtp)& transport can handle a number of
addresses containing a mixture of different domains provided they all resolve
&$domain$& in an expansion for the transport, because it is set only when there
is a single domain involved in a remote delivery.
+It is expanded per-address and can depend on any of
+&$address_data$&, &$domain_data$&, &$local_part_data$&,
+&$host$&, &$host_address$& and &$host_port$&.
.option port smtp string&!! "see below"
.cindex "port" "sending TCP/IP"