This seems like it could lead to you creating an open relay, which everyone (including spammers) can use to send spam.
e.g. a spammer buys a domain and sets the MX-record to your PMG-instance - with this config you would happily forward their spam to the world.
if you need to enter many domains at once - you can use the REST-API, or even add them to the config-files (/etc/pmg/domains, /etc/pmg/transports (make sure to run the appropriate postmap afterwards) )
Hope this helps!