Please always post the version of Outlook you are using and any other
relevant information with your questions.
The folder archive settings are stored in a hidden message in the folder
with a MessageClass of "IPC.MS.Outlook.AgingProperties". If you are using
Outlook 2007 you can use the Folder.GetStorage() method to access the hidden
item as a StorageItem. If you are using an earlier version of Outlook you
will need to use an alternate API such as CDO 1.21 or Extended MAPI (not
supported in managed code and in the case of Extended MAPI only usable from
C++ or Delphi), or a MAPI wrapper such as Redemption
(
www.dimastr.com/redemption).
Look at
http://www.cdolive.com/cdo10.htm which has the properties used for
archive settings and a link to some CDO code that works with the archive
settings. You'd have to translate that into C# and modify the code to work
with something like Redemption or with Outlook 2007.