mac.customcas.mobi has no MX records.
Question and response
Question
dig mac.customcas.mobi. MX
Answer
The response has no answer records.
Authority
customcas.mobi. 3601 SOA dns1.registrar-servers.com. hostmaster.registrar-servers.com. 1771499968 43200 3600 604800 3601
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0