Internet-gw.customer.alter.net has no MX records.
Question and response
Question
dig Internet-gw.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