123-reg's servers are configured to check to see if email is coming from a Smart Host server, this means that we check that the server we are receiving the email from is authenticated to send email as coming from that domain.
Example: If you were to send an email from user@ISP-Provider-UK.com through the SMTP server mail.ISP-Provider-UK.com, to a domain using our servers we will check if user@ISP-Provider-UK.com is authorised to send through mail.ISP-Provider-UK.com. If the domain is authorised to send through the server we will accept the email, if however the email hasn't gone through a Smart Host server and came from an Open Relay server we will block the email, as this method is widely used by Spammers.