forward500d.mail.yandex.net has no MX records.
Question and response
Question
dig forward500d.mail.yandex.net. MX
Answer
The response has no answer records.
Authority
mail.yandex.net. 300 SOA ns3.yandex.ru. sysadmin.yandex.ru. 1772124252 900 600 3600000 300
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0