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While most domains have just one "MX" record, some domains can have multiple MX records. The MX records might then be:
example.com. MX priority=5 example-com.relay1a.spamh.com.
example.com. MX priority=6 example-com.relay1b.spamh.com.
example.com. MX priority=10 mail1.bighost.net.
example.com. MX priority=20 example-com.relay1c.spamh.com.
When a mail server sends email to your domain, it first attempts to send it according to the MX record with the highest (lowest numerically) priority. If the two servers fail to establish a connection, the sending mail server tries the next highest priority MX record, until it goes through all of the MX records.
