In the latest version of SmarterMail 16, the spool doesn't refresh automatically, are you attempting to open a message that has already been delivered ? if so, when you attempt to open the message, a call will be made to load the EML file from the Spool, if this file has already been delivered, the content cannot display message is to be expected.
Before loading a message that's been known to fail to load the content, try refreshing the spool to see if the message goes away. If it does it has been delivered, if it's still present and throwing the error, please check the status of the message. If it is set to 'Receiving' the message is still being received and may not be viewable at that time.
I hope this helps.
Technical Support Supervisor