This FAQ will explain what MX Records are
The below article outlines how MX record work, what they're used for, and some of the practical applications for their use.
MX records or mail exchange records specify which mail servers are responsible for a particular domain. This entry is not mandatory, and – as with the A record – several entries may be made.
A MX record is a type of DNS record which is responsible for accepting incoming email and routing it to a specified mail server, based on the information in the record.
One special feature is priority numbers, which provide information to the querying mail server about which mail server should be used first. The next entry is tried only when the mail server with the highest priority is not available. The priority of priority numbers rises in opposition to the value of the number, meaning that lower numbers have a higher priority.
Another special feature is that only domains may be entered for mail servers. An entry referencing an IP results in an error.
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.
For 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.