MS Outlook invokes multi-kinds of error message on different-different situations and the error OST cannot be accessed because it has been configured for use with different mailbox is one of them. In a certain time, this error appears on your screen and blocks the OST file for use. Look at below some of the reasons
NSF to PST Converter
Single tool for Lotus Notes to Convert unlimited NSF file to PST and 8+ other conversion formats. This tool supports all versions of IBM Lotus Notes.