mail.stone-is.net has no MX records.
Question and response
Question
dig mail.stone-is.net. MX
Answer
The response has no answer records.
Authority
stone-is.net. 3600 SOA ns3.combell.net. hostmaster.stone-is.net. 1768479390 10800 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0