I have a large number of emails in my Rackspace inbox (~560,000 messages totaling 10GB). It wasn't maintained because all mail was being forwarded to a Gmail account (forwarded, while keeping the local copy). I would like to delete a lot of these (old cron emails) but I am not able to load my entire account into most email clients using IMAP (don't know if this is related to my network connection, slow computer, or Outlook itself). Additionally, the webmail client restricts me to deleting only 100 messages at a time. Note: new emails matching the remove criteria are being deleted by a incoming mail filter I setup, but I do not have an option to run that rule on existing messages.
What options do I have for cleaning up the existing messages in my mailbox?