Hi,
This notification is caused by a corrupted email that has bad base64 encoding, or has corrupted Multi-Purpose Internet Mail Extensions (MIME) parts, and cannot be scanned properly by the CASE. The CASE itself works as designed, and this is not an indication of an overall processing issue on the appliance.
If you observe these errors for a lot of emails you may want to open a TAC case with the original email sample for review.
Regards,
Libin