nsauth3.introweb.net has no MX records.
Question and response
Question
dig nsauth3.introweb.net. MX
Answer
The response has no answer records.
Authority
introweb.net. 3597 SOA nsauth1.introweb.nl. postmaster.introweb.nl. 2018041601 28800 7200 604800 86400
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0