smtp-01.servidoresdns.net has no MX records.
Question and response
Question
dig smtp-01.servidoresdns.net. MX
Answer
The response has no answer records.
Authority
servidoresdns.net. 3600 SOA atlante.servidoresdns.net. hostmaster.arsys.es. 2005152356 86400 7200 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0