If you are using a third-party / registrar DNS service then you must also configure your local instance of BIND to use Google MX via Virtualmin.
Additionally you might have to tweak the Postfix config file to add
sender_dependent_relayhost_maps = hash:/etc/postfix/relayhost_maps
and then create a relayhost_maps file and add something like
@eduwebsite.edu [xxx.xxx.xxx.xxx]:587
and then
postmap /etc/postfix/relayhost_maps
and restart Postfix