As I understand it, OE stores messages within a database, which tends to grow in size and degrade the perfomance of OE. According to one source I checked, even deleted messages that are removed from the folder take up space in that database. Compacting your folders will REMOVE all the records that you have deleted.
I do not know why this message is now appearing, unless you changed something in OE. I regulary compact messages in Email folders.
Hope this helps,
Edited by jgweed, 30 January 2005 - 12:05 AM.