Let me see if I understand your setup. You're using Outlook Web Access to
access your Exchange mailbox, and in the office, your Exchange server is not
handling Internet mail, only internal mail. In the office, you use Internet
Mail on your Outlook client to access your POP mail from your ISP and
download it into your Exchange mailbox.
Is that correct?
If so, you cannot use Internet mail at all in your Outlook Web Access -
Exchange doesn't have any way to handle it. You can't use POP in OWA at all,
ever. I would suggest speaking to your Exchange admins in the office and
suggest to them that they set up Exchange to host your company's domain's
Internet mail. It is not difficult to do.