BT Business Broadband Email Settings
As BT don’t seem to publish this information, or at least without major trawling of this, I thought I’d make a note of it.
For STMP you have to use authentication (not secure-auth) that is your username and password must be sent. The username is the bit before the @btconnect.com of your email address, not the whole address as the BT docs say. Its your account credentials, not your router credentials which are btclick.