Hello glen, welcome to PCHF.
You can only send out via the BT SMTP server, using their SMTP address, which used to be either
mail.btinternet.com or
relay.btinternet.com.
If your unsure on how to change this, hopefully this document will help:
SMTP For NonBT Email (make sure that you only use your BT address for outbound e-mail only).
EDIT: Marked as Answered.