I have an exchange 2010/outlook 2010 environment, and I have a number of users that have multiple mailboxes set up in their Outlook. Each mailbox often has its inbox with dozens and dozens of subfolders set up. Sometimes when they add a subfolder the subfolder disappears and I then have to delete their profile and ost and rebuild it, and then it works. The issue at hand is that some of these ost files are 35-40 Gb in size. Yes, that is correct. Many of the emails have pictures and or video clips as attachments, and the mailbox size can grow rapidly. My concern is the size of these ost files. Is there a problem having such large ost files? Is there a way to keep these sizes down?