Andrew:
You can put your organization SMTP server, which has sending capabilities.
Following are the steps you have to follow( This is from Rob)
From the RTMT (Real Time Monitoring Tool) Have a look;
Configuring E-mails for Alert Notification
The following procedure describes how to configure e-mails for alert notification.
Procedure
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Step 1 In the left controlling center pane, click the Alert tab.
Step 2 Click Alert Central.
The Alert Central monitoring window displays and shows the alert status and alert history of the alerts that RTMT generated in the Cisco CallManager cluster.
Step 3 From the RTMT menu bar at the top of the pane, choose Alert > Config Email Server.
The Mail Server Configuration window displays
Step 4 In the Mail Server field, enter the e-mail recipient information.
Step 5 In the Port field, enter the port number of the recipient.
Step 6 Click OK.
Also the document
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/service/4_1_3/ccmsrva/sartmt.html#wp1257958
HTH