iCloud email sort by date not working

I use the Mail.app on my Mac for both my Gmail and iCloud email accounts.

I recently copied my Gmail emails over to the Archive folder of my iCloud account (so I could disable Gmail and still have those emails available). The copy appeared to work fine - all emails copied over are now in the iCloud Archive folder as expected. However, the sort order of those messages is not working properly.

The messages appear to be sorting in the order in which they were copied from the Gmail server over to the iCloud server, rather than based on the actual sent date of the message.

For example, one of the emails copied over had an original sent date of 12/2/2012. In the preview version of the message the date shows as 'Yesterday'--the date when I copied that email into iCloud. However, when I open the email and look at the sent date it correctly displays as '12/2/2012'.

Unfortunately, the sort by date function appears to be sorting based on a hidden timestamp related to when the message was written to the server, rather than the actual sent date of the message.

Is there any workaround to get the emails to sort as one would reasonably expect (by sent date)?