If the original message is readable in the user interface (such as in the Incident Detail page), but unreadable in the SMTP notification, the issue is that the Incident Persister service was unable to decrypt the message before sending the notification. The original message is stored encrypted in the Oracle database, but in some cases, the encryption keys stored by the Incident Persister become out of date. The Manager service still has the correct keys, so you can still read the message in the user interface.
You can resolve this problem by recycling the services on the Enforce server; this will cause the services to refresh the keys automatically. After the services restart, any new notifications will have the correct original message attached.