listen
Specifies the local IP and port number to which the ESMTP service should bind and listen.
Use 0.0.0.0
to bind to all IPv4 addresses.
Note
This option cannot be used in dynamic listener contexts such as within
via, peer or within the parameters returned from
smtp_server_get_dynamic_parameters.
It can only be used directly at the top level within the
kumo.start_esmtp_listener
call.