MX records explained

• An MX record is an entry in a domain name database that identifies the mail server that is responsible for handling e-mails for that domain name.
• When more than one MX record is entered for any single domain name that is using more than one mail server, the MX record can be prioritized with a preference number that indicates the order in which the mail servers should be used. This enables the use of primary and backup mail servers.

Example of an invalid MX record: (RFC 1035 record should not specify an IP address)
nslookup -type=mx exampledomain.com
Server: dcsfm1.internal.salesforce.com
Address: 10.1.11.2

Non-authoritative answer:
exampledomain.com MX preference = 10, mail exchanger = 201.46.7.135

Example of a valid MX record:
nslookup -type=mx salesforce.com
Server: dcsfm1.internal.salesforce.com
Address: 10.1.11.2

Non-authoritative answer:
salesforce.com MX preference = 300, mail exchanger = salesforce.com.s8b1.psmtp.com
salesforce.com MX preference = 200, mail exchanger = salesforce.com.s8a2.psmtp.com
salesforce.com MX preference = 100, mail exchanger = salesforce.com.s8a1.psmtp.com
salesforce.com MX preference = 400, mail exchanger = salesforce.com.s8b2.psmtp.com

salesforce.com.s8b1.psmtp.com internet address = 64.18.7.13
salesforce.com.s8a2.psmtp.com internet address = 64.18.7.11
salesforce.com.s8a1.psmtp.com internet address = 64.18.7.10
salesforce.com.s8b2.psmtp.com internet address = 64.18.7.14

Notes: 

• When we deliver mail, we always try to deliver to the MX record with the lowest preference.
• It is also required that the 'exchange' value which is tied to the MX record has a valid 'name' and that it DOES NOT POINT TO AN IP ADDRESS.
• There are many websites that can be used to perform lookups, such as http://network-tools.com/