Not sure about the error message, but if it keeps getting the same 3 messages then the 4th would logically be the one causing a problem.
You could use your Zimbra Admin console to view the account and then "View Mail". Look at the next message in line via your web client and delete if necessary, if it is SPAM or something.
Finding out why that 4th message down is causing a problem is beyond me.
"Let's look at this from a standpoint of "Status". What exactly, on the Space Craft, IS working?"
-Flight Control, Apollo 13