1. Disable email scanning by your anti-virus application.  It provides no
additional protection, it may be causing the problem, and even Symantec says
it's not necessary:

<quote>
Disabling email protection does not leave you vulnerable to viruses and
malicious software in email. It is a separate layer of protection in
addition to Auto-Protect. Auto-Protect scans any incoming files, including
email, as they are saved to your hard drive. As long as you keep your virus
definitions up to date with LiveUpdate, and keep Auto-Protect enabled and
set to scan files as they are created or downloaded, your system is fully
protected.
</quote>
From the Symantec page http://snipurl.com/bmf6

2. Troubleshooting error messages that you receive when you are using OL and
OE
http://support.microsoft.com/?kbid=813514

3. Your current Identity may be damaged, especially if its the default Main
Identity.  Try one or more of your accounts in a new Named Identity
(File>Identities>Add new identity).  Assuming all is well, import messages
from the old Identity before deleting it (File>Identities>Manage
Identities).

OE Caveats:

Don't use Inbox or Sent Items to archive messages.  Move them to local
folders created for this purpose.

Empty Deleted Items folder daily.

Disable Background Compacting [N/A in SP2] and frequently perform a manual
compact of all OE folders while "working offline".  More at
http://www.insideoe.com/files/maintain.htm.

Your anti-virus application's email scanning feature can also cause
corruption.  Disable it.  It provides no additional protection.

PS: OE6-specific newsgroup:
news://msnews.microsoft.com/microsoft.public.windows.inetexplorer.ie6_outlookexpress

Try unchecking tools>accounts>mail>properties>advanced>this server
requires a secure connection.

Kath