The LastModifiedTime and CreationTime properties in Outlook are read-only.
If you use Extended MAPI/CDO 1.21/Redemption, apparently you can modify the
PR_CLIENT_SUBMIT_TIME and PR_MESSAGE_DELIVERY_TIME properties, but
PR_CREATION_TIME and PR_LAST_MODIFICATION_TIME (that map to the
aforementioned Outlook properties) are still read-only.
I'm not sure if there are any fields exposed through Outlook in a view that
map to the PR_CLIENT_SUBMIT_TIME or PR_MESSAGE_DELIVERY_TIME properties, so
it may not be helpful to modify those properties anyway (if you indeed can -
I'm still not 100% sure).