you just set up an IMAP account for your BT email address and use the correct server setting shown here
Do you mean the webmail Outlook.com or do you mean the email client Outlook?
Thank you for the update.
Email client, but as the BT acc has no limit, that is not a problem and apparently Outlook is 15gb then I'll stay as I am for now and just trim the BT acc down now and then. It is useful if I mange to lose something in Outlook - I know it will still be there in the BT mail acc.
If you have your Outlook client set up as an IMAP account, if you delete anything from Outlook it will be deleted from BT and vice versa because ithey synch with each other.
You will need to set up your Outlook client as a POP3 account if you do notwant it deleted the BT account's emails when you delete them in Outlook.
POP3 does not synch with the BT servers except to download the Inbox.