You must have your Outlook set up as an IMAP account. This means that the BTMail is synchronised so what you do on one device is replicated on the other device.
You would need to delete your present Outlook account and reset it up as a POP3 account so that it could download your emails from the BT servers but it would not synchronise and as such would not replicate what each of your devices are doing.
See links
BT Email server settings | BT Help
Email: What's the Difference Between POP3, IMAP, and Exchange? (howtogeek.com)
