Mail Exchange (MX) Records

MX records are a type of DNS record that maps one host name to another. Equal priority MX records route messages to redundant inbound MTAs. For example, the sending MTA from the Internet will find that the MX record for siroe.com corresponds to MTAA.siroe.com and MTAB.siroe.com. One of these MTAs is chosen at random, as they have equal priority, and an SMTP connection is opened. If the first MTA chosen does not respond, the mail goes to the other MTA. See the following MX record example:

siroe.com. in MX 10 MTAA.siroe.com
siroe.com. in MX 10 MTAB.siroe.com