Google Email Uploader: upload email from your Mac to Google Apps
The biggest caveat here is that we're not talking about uploading to a Gmail account. Google Email Uploader only works with Google Apps email (i.e. POP3 and IMAP). Other than that, it couldn't be easier to upload messages from your Mac - assuming you're running a popular mail client like Apple Mail, Eudora or Thunderbird. A warning, though: due to server rate limits, you might want to upload 500 messages or fewer at a time, because performance will lag significantly on large batches.