The IE thing was just to figure out the error message…
Try to access the page by adding this parameter: content/plugins/postie/get_mail.php?Submit=Run+Postie
Please, tell us how did you solve this situation.
This has happened with my blog too. I don’t know how the plugin checks if the message shall be post or a reply… Did you change the post title or something like that after the replies?
Try to disable “Show friendly HTTP error message” from IE > Tools > Options > Advanced. Then you’ll be able to see the error. 🙂
