Not receiving all emails from Telstra Exchange Server

[RicDav]

I have a small business with 3 PCs and 1 BlackBerry. Most of our business is via email. I have an 8700g which worked fine on Big Pond POP account, BIS plan. Telstra advised me to change to Business Broadband and Business Email because I needed a bigger mailbox and to share calendars. When I changed to IMAP account on Telstra Exchange Server, Outlook worked fine but got no emails on the BlackBerry. Telstra no help so I found how to change the settings to IMAP on the RIM website but only recieve about a third of the emails coming in. Back to RIM website, find a suggestion to turn off cached exchange mode, have done this but it has made no difference. Telstra have told me many different things, from I need a BES plan, to it's impossible to connect the BlackBerry to the Exchange Server, which surely can't be true.
Can anyone clear this up for me? What are my options?

[GaryCutri]

On my personal BlackBerry (Telstra) I have it connected to my Exchange server account and I can get all my emails. The most common cause of missing emails with your configuration is Outlook rules. If you have any rules that automatically file items into subfolders this will cause the items to be removed from the inbox, this in turn causes it to be missed by the Blackberry next time it scans your mailbox.

[RicDav]

re not receiving all emails on Exchange Server: Thanks for your reply, I have no rules in Outlook!

[RicDav]

After several days changing various settings, when we changed "Keep Messages For" in options from "Forever" to "30 Days" it deleted a huge archive which presumably was the problem so now we receive all emails BUT, new problem: when we reply to an email in Outlook, the original email we are replying to is sent to the BlackBerry. ie we have two copies on the BlackBerry of anything we reply to which is really irritating since we reply to most emails. Any thoughts on the cause and/or solution?

[GaryCutri]

Have you considered changing to hosted BlackBerry Enterprise Server (BES) now that you have hosted Exchange? If possible can you please delete the email settings from one of the BIS accounts and then reconfigure them from scratch but this time select the option to use "Outlook Web Access"?