A resource record, which is used to specify a mail server in Domain Name System (DNS) that actually receives email messages on behalf of the domain of that recipient, is known as MX record. MX record is actually the abbreviation of mail exchanger record. A preference value is also used by this resource record which actually sets the deliver priority of emails if multiple mail servers are in use.
MX records are actually used in a set just to specify the route of email delivery and receiving for a domain and Simple Mail Transport Protocol is also used to complete the process. Preference number, which is actually presented in the MX record of recipient domain’s DNS, determines the related priority the MX server for every SMTP mail request.
When MX lookup is being done by the remote client (normally it is not the same mail server) for the domain name, a list of servers returns to it along with the preference numbers. As per the rule, highest priority is being given to the smallest number and first request is always sent to server who has the smallest preference number. If there are more than one mail servers that are having the same smallest preference number then first try will be given to all those servers before generating request to other servers with lower priority.
Spammers normally use to get connection with the high distance MX servers that are normally known as back MX servers of any domain first of all in intention to avoid filtering their emails by anti-spam technique of the server as these servers normally use highest preference numbers.
more:here
