Whether you have a UK cPanel hosting or UK email hosting account with Farbyte, in order to send mail through Farbyte's mail servers, your email client program must be configured to use SMTP Authentication.
Common email clients include a checkbox in the account configuration with the label "This server requires authentication".
That box should be ticked.
This error can also occur with some ISPs which block port 25 for SMTP.
AOL is known to be such an ISP.
