Postfix will not send mail for asterisk

I have been trying for 6 hours to get asterisk to send mail through postfix. When I use the mail command line, e-mail sends fine. When asterisk sends a voicemail notification, it never makes it through. It bounces with 550 Administrative prohibition. I think it is because the message-id being sent by Asterisk has the machine hostname and not the domain name of the smtp server. How can I fix this?

Using Asterisk 11 and latest freepbx

I have verified this is the problem by changing the hostname of the machine temporarily. Asterisk uses the message id *****@hostname to send e-mail. My SMTP server that is being relayed to will only accept mail with a message-id that reads like How can I make Asterisk send the proper message-id?