- Setup Mailbox in Mail.app
- Allow mailbox to download, including subfolders like Sent Mail
- Close Mail and rename your Envelope Index database (mv ~/Library/Mail/Envelope Index ~/Library/Mail/Envelope Index.bak). Open Mail again – you will be presented with an Import dialogue. Mail will rebuild your index, including getting the received times of your sent emails from the email headers.
Taken from this thread: http://www.gabrielserafini.com/blog/2006/12/18/fixing-mailapps-imap-date-problem-mostlygeek/