Reg:Postfix Mail Server

We have a redhat enterprise linux 4 server.we have configured postfix as our MTA.squirrel mail as webmail and apache is configured for webserver.We configured outlook as mail client.The mail server is working and our users ae saving a copy of mail in the linux server.Our problem is whenever the users access and close the webmail,the outlook will download all the mails eventhough it has downloaded before.kindly need solution for this.waiting for ur favorable reply.

With Regards,

Ramji