We have a practice wide re-occuring issue in which users .ost files become corrupt. See error: "Unable to open default email folders. The file C:\\microsoft\outlook\outlook.ost is not an offline folder." We are on Outlook 2007 (12.0.6423.1000 SP2 MSO), office 2007. Occurs on different PC's & laptops, happens on 32-bit and 64-bit OS. We know that deleting the .ost file or re-creating the mail profile will fix this issue, however, we have numerous Users around the world, which this is not an option. Moreover, this is a re-occuring issue and want to get to the problem rather than the work around. Advice and Suggestions are welcome!