The technologies that allow you to download the email messages you receive to your device are called IMAP and POP. In order to know which of the two you are using, open the program for you email, highlight the account about which you wish to know and then open up its properties or its settings. Here, check whether the Mail server type is POP or IMAP.
Because IMAP is always synced to your BT Email server, all changes made to the email program appear in the webmail inbox, while POP does not sync to the BT Email server, so there is a high chance that the changes may be lost. Thus, for a regular user, it is recommended to use IMAP where Secure Socket Layer (SSL) is enabled. SSL makes sure all data that is exchanges is encrypted so that no one can steal your username or password. All emails sent through BT also needs to have SMTP authentication, which checks the source from where the email originated and does not allow them to be sent anonymously. This helps in putting up a hefty fight against spam. Although, you may have to manually enable it for your account.
The incoming mail server is mail.biternet.com, the port is 993, which is SSL Encryption enabled. Your username is your complete email address and your password is your bbtopenworld or btinternet password. The full stop works as the folder separator. There is no need to enable secure password authentication (SPA) and it likely will not work even if you di enable it.
The settings for this are same as the settings above, except that the Port is 995.
The settings for SMTP are also the same as above, except that the port is 465 and the authentication is PLAIN.