It's configured on the 'session target mailto:' under the mmoip dial-peer that gets matched when the inbound call arrives. For cisco Unity, we always send it to a specific mailbox, such that it can be checked and delivered to a user based on the subject line.
session target {mailto: {host-name | $d$. | $m$. }@ domain-name | ipv4: destination-address | dns: {$d$. | $e$. | $s$. | $u$. }host-name}
Example:
Router(config-dial-peer)# session target mailto:$d$@abccompany.com
See:
http://www.cisco.com/en/US/partner/products/sw/iosswrel/ps1839/products_feature_guide_chapter09186a00800b5dcf.html#wp1106037