I am guessing that you want to send notification to users on another exchange server correct?

To do this, in the unity subscriber, and I am trying to remember of the top of my head, find the menu for message notification.  In there will be an email setting. You can enter the email address here.

You will need to be sure that an SMTP gateway os configured first in order for this to work, otherwise an NDR will be generated.  Here is an excerpt from Cisco...

In order for subscribers to receive SMTP message notifications from Cisco Unity, your site must have
an SMTP gateway. When the gateway is set up, you can use the Text Pager 1 or Text Pager 2 options on
the Subscribers > Subscribers > Message Notification page to enable Cisco Unity to send text message
notifications by using SMTP. Alternatively, subscribers can enable the same devices on the Notification
Devices page in the Cisco Unity Assistant.
Note that when a site without an SMTP gateway attempts to deliver text message notifications, the
notification attempt fails and a nondelivery receipt (NDR) is sent to the Cisco Unity Messaging System
and then routed, by default, to the Unaddressed Messages distribution list