Your email will be checked AUTOMATICALLY every 12 - 15 minutes. Except for Yahoo, GMail and the BlackBerry.net email addresses, those are INSTANTLY delivered...
The cox.net email would be delivered every 15 minutes. But, today in particular, BIS / RIM have had issues, i.e. delays...
I believe BIS is working ok now, nothing you could do about it. Also, there is no way to change the "sync interval" either. If it needs to be instant, you can forward to your @carrier.blackberry.net address or a gmail account.
Did you go into your Outlook program and select the option to leave messages on the server?
That was a huge problem when we first set up our cox.net addresses on our BB's.