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That message means the phone is unable to connect to the specified server so it could be a problem with your internet connection or you have wrong serveraddress, port or encryption selected in the account.
I'm not sure if they use same settings for all users but i found this url: http://bt.custhelp.com/app/answers/detail/a_id/44917/~/bt-email%3A-what-are-the-settings-for-outgoin...
According to that you should enter:
Username: your emailaddress
Password: password
POP3 server: mail.btinternet.com
Security type: SSL/TLS (also try the one with accept all certificates)
Port: 995
If that doesn't work, have you tried this both over WiFi and mobile data?
