Presumably, you have two email accounts set-up in Outlook Express - one being BT and the other being Freeserve.
You currently can receive within OE, emails under BOTH accounts but can only SEND, using your BT account.
If you change the smtp server details in the Freeserve account to those of the BT account, you should then be able to send messages from your Freeserve address, within OE.
Why don't you send a message from your Freeserve address to your BT address to verify that it works ? Or even Freeserve to Freeserve.