Could be a few things, so let's start at the obvious.

Do not archive mail in default OE folders. They will eventually become
corrupt. Create your own user defined folders for storing mail and move your
mail to them. Empty Deleted Items folder regularly. Keep user created
folders under 100MB, and Default folders as empty as is feasible.

After you are done, follow up by compacting your folders manually while
working *offline* and do it often.

Click on Outlook Express at the top of the folder tree so no folders are
open. Then: FileWork Offline (or double click Working Online in the Status
Bar). File | Folder | Compact all folders. Don't touch anything until the
compacting is completed.

In Tools | Options | Maintenance: Uncheck Compact messages in background and
leave it unchecked. {N/A if running XP/SP2}.

Do this and let's see what happens.
--
Bruce Hagen
MS MVP - Outlook Express
~IB-CA~

"Tom Brown" wrote in message
...
My sister has a problem with emails that she has received disappearing
from her Inbox after she has replied to them. I don't think it is a Rule
because I think that happens before the download.

She is still trying to find a pattern but so far it looks like if she
replies to an incoming email, it disappears at some time after that. She
is able to see it again in the Sent folder.

Any clues?

TIA,

Tom