This is an odd one, because we're only seeing this one on occasion...
However, there is concern, because the error indicates that the message could not be written to disk!
Here's the full text of the error message:
"SMTP Protocol Returned a Permanent Error 554 Error writing message to safe storage; message could not be stored to disk."
This is the message that shows on an occasional dead mail message (incoming).
The problems seems all that more obscure, because the e-mails are not to a specific user, the e-mails are addressed to existing "working" addresses, and the error messages do not appear on any consistent basis at all. Even the "554" error seems inconsistent with normal 554 errors.