One of my email accounts is hosted at an ISP with unreliable IMAP support, and I can't change it. Fortunately, I have my personal email set up on Google Apps for Domains, so I created another GMail account there and turned on GMail's features that allow me to send and receive mail through the ISP account using GMail ("Send mail as" and "Get mail from other accounts" in GMail settings on the Accounts tab). I'm now using Outlook to retreive mail from the GMail account through IMAP, which in turn is retreiving mail from the ISP account through POP3.

This basically works great, except for one very significant issue:

Prior to setting this up, I already had several months of mail in the ISP account that I had been accessing via IMAP. GMail grabbed all of this mail via POP3 at, let's say, noon on April 5. In GMail's web interface (and on my iPod touch, and in Mozilla Thunderbird), all is well: the messages are all shown with their original time stamps. But when Outlook downloads these messages from GMail via IMAP, the time stamps are all set to noon on April 5 (the time GMail downloaded them from the ISP via POP3). That's not good, especially since we're talking about hundreds of messages here over a time span of several months.

How can I fix this and get Outlook to display the original time stamps?