If you don't want change/configure postfix - I may suggest to use swaks. It's a perl script which has a lot of functionality. I'm using the following command with nagios without any problem
/usr/bin/printf "%b" "$MESSAGE\n" | /usr/bin/swaks -4 --server
smtp.googlemail.com:587 --from $USER3$ --to $CONTACTEMAIL$ -tls --auth PLAIN --auth-user $USER3$ --auth-password $USER4$ --h-Subject "$SUBJ" --body - --hide-all