The PureMessage Activity Monitor or ExchangeStoreScan.log contains the below error:
PureMessage store scanning has encountered an application error (details: 'Unknown error')
First seen in
PureMessage for Microsoft Exchange 4.0
The Exchange Web Services (EWS) based store scan is attempting to access an unrequired item and causing a NullReferenceException error.
What To Do
Download and install PureMessage for Exchange version 4.0.2 which is not affected by this issue.