You best option is to look at the traces as indicated in Unity connection Troubleshooting guide.
http://www.cisco.com/en/US/docs/voice_ip_comm/connection/8x/troubleshooting/guide/8xcuctsg160.html#wp1062323
SMTP Message Notification Is Not Working at All for Multiple Users
If SMTP notifications are not working, in Cisco Unity Connection Administration, check the System Settings > SMTP Configuration > Smart Host page to confirm that a smart host is configured. To enable Connection to send text message notifications by using SMTP, your Connection server must be configured to relay messages through a smart host.
If a smart host is already configured on the Smart Host page, note the IP address or host name of the smart host and check to make sure that this smart host is configured to accept messages from the Connection server.
If the smart host settings are configured correctly, you can use traces to track whether the SMTP notification messages are being sent by the Connection server. The default SMTP micro traces (levels 10, 11, 12 and 13) indicate if there is a permanent problem with delivery of a notification message to the smart host. The SMTP micro trace level 18 (Network Messages) shows the details if the notification message is delivered to the smart host. For detailed instructions on enabling and collecting diagnostic traces, see the "Diagnostic Traces in Cisco Unity Connection 8.x" chapter.
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