An interesting post from the Outlook Team blog about the Data File checks and Outlook client.
When Outlook doesn't shutdown cleanly, your Outlook Data File (either a .pst or .ost file) can be left in an unverified state that can cause one of the following errors to appear the next time you boot Outlook:
"Outlook cannot open the data file filename until it has been checked for problems."
"Microsoft Office Outlook' exited without properly closing your Outlook data file 'drive:\Users\username\AppData\Local\Microsoft\Outlook\outlook.ost'. 'Microsoft Office Outlook' must be restarted. If this error message recurs, contact support for 'Microsoft Office Outlook' for assistance."
"The data file filename was not closed properly'. This file is being checked for problems."
Read more at source: http://blogs.msdn.com/outlook/archive/2008/05/08/data-file-checks-and-you.aspx