In testing I've done, the modified date is updated on a .PST file just by
opening and closing outlook.  Some info I've found on a newsgroup stated:

"PST files initiate a locking mechanism when you access them (access, not
necessarily open). A PST file is a database. This database is continuously
interrogated with searches and sorts by Outlook to get the data it needs to
display, import,... whatever. There is a special reserved area in the
database Outlook writes to when it accesses the file. Information about the
user and computer currently accessing the file is placed here. This allows
other processes on that particular computer (invisible queries to build the
folder hierarchy in the Outlook Folder List, for example) to access the file
while effectively locking-out access to anyone else until the file is
closed. Without this exclusive access, a PST could be easily corrupted by
someone changing parts of that database at the same time. Without the
ability to record in the PST file itself who has access to it right now,
there is no way to lock out others while allowing the "right" person
access."

So along with breaking up the .PST files, you may also have to make them
read-only.   Some of our users have setup a separate archive pst for each
year, then they are only modifying the most recent one.

Tim Rushfoth
City of Winnipeg