You could use your own domain's server settings or you could send emails using the BT SMTP server mail.btinternet.com, port 465, SSL ticked and authenticate with your firstname.lastname@example.org email address and password.
Thanks to you all. I have authentcated with my email address, but where do I enter my password ?
Are you using your own domains settings or BT's settings?
Depending which email client you are using there should be a tick box to "require authentication" and a settings button.
If you are using BT's servers, click on that then enter your BT email address and password. Don't use the " Use same as incoming"
If you are using your domains settings you could just select "use same as incoming".