You can change/set the From-addresses in FreePBX to something you want.

System notifications

Change the address in Admin » System Admin. To the right, click on Notification Settings, then fill in the From Address.

Voicemail notifications

These are set up from within Asterisk, and are located at Settings » Voicemail Admin. Click on the Settings link, then scroll down until you find serveremail. Change it to whatever you want.

Next, because it uses Postfix to e-mail out voicemail notifications, you'll also have to change a parameter in there:

1. Open a shell to your box (SSH).
2. Type nano /etc/postfix/main.cf.
3. Find and change, or add a line: mydomain your-domain-name.com. Fill in the domain you want. It should be the same as you set for voicemail notifications.
4. Restart Postfix: service postfix reload or postfix reload.