Why does Windows Mail fail to find the host 'btopenworld'?

[babs48]

I receive the following message when I try to send an email via Windows Mail.
"The host btopenworld could not be found. Please verify that you have entered the server name correctly.
Subject 'winkworth', account: POP3, server: 'btopenworld', protocol: SMTP, port: 25, secure (SSL): no, socket error: 11001, error number: 0x800CCC0D"
Please help me to resolve it. Many thanks.