Postfix From Address

Sorry to have to ask this question, but I can't figure out where to change the asterisk@ setting in Postfix (at least I'm assuming it's a Postfix setting). Of course, this may be an Asterisk setting. Anyway, here is my problem:

I'm trying to send voice mail notifications through an Exchange server. However, Postfix insists on sending asterisk@ as the "sender" email address. Here is a copy of relevant info from /var/log/maillog when the system attempts to send a voicemail notification email:

-----start snip-----
Aug 31 20:07:17 voice postfix/cleanup[5301]: DEE58BB087B: message-id=
Aug 31 20:07:17 voice postfix/qmgr[4926]: DEE58BB087B: from=, size=11533, nrcpt=1 (queue active)
Aug 31 20:07:17 voice postfix/smtp[5304]: certificate verification failed for legolas.pcca.local: num=20:unable to get local issuer certificate
Aug 31 20:07:17 voice postfix/smtp[5304]: certificate verification failed for legolas.pcca.local: num=21:unable to verify the first certificate
Aug 31 20:07:18 voice postfix/smtp[5304]: DEE58BB087B: to=, relay=legolas.pcca.local[10.2.1.22]:25, delay=0.12, delays=0.04/0.01/0.05/0.03, dsn=5.7.1, status=bounced (host legolas.pcca.local[10.2.1.22] said: 550 5.7.1 Client does not have permissions to send as this sender (in reply to end of DATA command))
Aug 31 20:07:18 voice postfix/cleanup[5301]: 02989BB087C: message-id=
Aug 31 20:07:18 voice postfix/qmgr[4926]: 02989BB087C: from=, size=13558, nrcpt=1 (queue active)
Aug 31 20:07:18 voice postfix/bounce[5306]: DEE58BB087B: sender non-delivery notification: 02989BB087C
Aug 31 20:07:18 voice postfix/qmgr[4926]: DEE58BB087B: removed
-----end snip-----

The TO address is the only thing I change when posting the output of /var/log/maillog

Legolas (the Exchange server) is rejecting because it's attempting to send as asterisk@voice.pcca.local. I need it to send as voice@domain.com. I can change it to asterisk@domain.com by changing myorigin in mail.cf, but I can't change the asterisk@ to voice@.

Thanks.
--
Josh