I found this very excellent how-to that works perfectly with AsteriskNow, and thought I'd pass it along.
It describes how to use Cyrus SASL with Postfix to allow Postfix to authenticate you as an authorized user of that email server, to allow email relay. Cyrus SASL is part of the distribution for AsteriskNow.
This will allow you to deliver voicemail through your regular email providers server. Normally, an ISP email server will not allow email relay to happen, as that is a favorite tactic of spammers.
http://postfix.state-of-mind.de/patrick.koetter/smtpauth/smtp_auth_mailservers.html