su has no MX records.
Question and response
Question
dig su. MX
Answer
The response has no answer records.
Authority
su. 3600 SOA a.dns.ripn.net. hostmaster.ripn.net. 650208235 86400 14400 2592000 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0