Symantec Messaging Gateway (hereafter SMG) processes messages in-memory if they are under 512KB in size. However, for these over 512KB, SMG writes the oversized chunks onto the temporaly disk space first, then scan them.
After the scan, the in-memory contents and the corresponding on-disk contents are to be reconstructed as a single message for delivery.
SMG has a managemental issue with these on-disk contents and under certain exceptional circumstances it mixes contents from different messages and / or the set of recipients.
When SMG is under heavy load and messages are backing up in the processing queues, on a very rare occasion, two of these on-disk temporary file names collide with each other.
This issue has been addressed in SMG 9.5.4 and the later. Please update SMG to the latest version.
This issue is confirmed to occur under the following conditions.