customer.alter.net has no MX records.
Question and response
Question
dig customer.alter.net. MX
Answer
The response has no answer records.
Authority
customer.alter.net. 21600 SOA auth02.ns.uu.net. hostmaster.UU.NET. 9125967 21601 3600 1728000 21600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0