Microsoft Exchange

I am frustrated with my Microsoft Exchanger Server. I use Microsoft Small Business Server 2003. We have about 25 mailboxes, to be downloaded , set every 15 minutes.
There was one mailbox, that the emails were downloaded but could not reach the mailbox in the server. So, I lost all the emails, since they had been deleted automatically after downloaded. Then I tried using PopCon for downloading. In the beginning it run well, seems everything was okay. But now, something worse come out. Emails from two mailboxes do not reach the mailbox in server.
Anybody can help me?