MX Records for Aliased Domain

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How do I add MX records for an aliased domain?

Add the MX records to the DNS for the SOA (Start Of Authority) responsible for the aliased domain. For example, if the aliased domain is sample.com and the SOA is bigisp.net, add the MX records to the DNS configuration of bigisp.net. How that is done is 100 percent dependent upon what DNS software bigisp.net uses.

The person who registerd sample.com had to designate a primary DNS, which will be the SOA for the domain. Additionally, the person should have also registered a secondary DNS. The administrator for the SOA then updates their DNS to recognize sample.com and serve out the necessary information, in this case, MX records. If there is a secondary DNS--as there should be--then the administrator for the secondary should update his DNS to get zone transfers from the primary, which includes the information for sample.com. And the administrator for the primary DNS should update his DNS to allow zone transfers to the secondary. Usually the administrator for the primary DNS tells you who you should select as the secondary.

Section 5 of RFC 2821 describes the semantics of MX records in detail.