If you're running 16.3.6467, will you please do this:
1. Stop the mailservice
2. Go to mailConfig.xml and add this:
3. Restart the mailService
This will enable some logging that we can then review to troubleshoot the issue.
When you see it occur, send a ticket to our Sales team -- you can put it to my attention -- attach the log file and let us know the EXACT time the issue occurred, and I'll pass the info along to the developers. The log file you'd look for is [date]-debugLog.log, where Date is that day's date.