So then I suppose you're using POP3. With POP3 each client downloads a copy of the (new) messages off the server, optionally deleting them afterwards.

With IMAP each client (Outlook, N900) has a "view" of the messages as they are on the server. This way multiple clients can concurrently access the server, and all of them see exactly the same thing. So if you delete an e-mail on the N900, it will not show on Outlook, etc.

Check if your "btinternet.com" account offers IMAP access. If so, change your settings both in Outlook and in your N900 to use IMAP instead.