These old emails were dragged out of Outlook Express to disk on a Windows Xp system, then copied into folders on this Windows 7 PC.
I have not imported these old emails into WLM. But when I open the folder and hover over them the Date Modified changes.
If I import one into WLM then its date in there is correct. But it means that I cannot easily see the chronology in Explorer, so tracking down the right one can be difficult. All was fine in XP/Outlook Express.
Your reply prompted me to do a bit more investigation - there is a "Date Received" column in Explorer - and that lists the original date.
(And it's a search term too)