OK, recently upgraded to Outlook 2013. After a few days I noticed several new 'Data Files', that I don't ever remember being there in my older 2007 version. In checking them out, they are somehow conncected to my work IMAP account, although it's primary Data File is an ".ost" file. There are four (4) of these ".nst" files now, all about 16 GB in size and I see no apparent reason for them, other than taking up valuable hard drive space. I've tried deleting them, but they just come back, again and again and again. Have searched for other methods to eliminate them, but cannot find anything.
"It's your data file - either for notes or for the outlook hotmail connector. Because its an offline message store, if you delete it, outlook will recreate it. "
Help appreciated.