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About MX Records
When an e-mail message is sent through the Internet, the sending mail transfer agent makes a DNS query requesting the MX record for the recipient's domain name, which is the portion of the e-mail address following the "@". This query returns a list of host names of mail exchange servers accepting incoming mail for that domain, together with a preference number. The sending agent then attempts to establish an SMTP connection to one of these servers, starting with the one with the smallest preference number, delivering the message to the first server with which a connection can be made. If no MX records were present, a second request is made for the A record of the domain instead.
The MX mechanism provides the ability to run multiple mail servers for a single domain and the order in which they should be tried, increasing the likelihood that mail may be delivered and providing the ability to distribute the processing of incoming mail across multiple physical servers.
The MX mechanism does not grant the ability to provide mail service on alternative ports, nor does it provide the ability to distribute mail delivery across a set of equal-priority mail servers by assigning a weighting value to each one. As of 2004, some MTAs, most notably exim, now support the use of SRV records for publishing the IP addresses, ports, priority, and weights of mail servers.
Source: wikipedia.org
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