unknown.yahoo.com has no MX records.
Question and response
Question
dig unknown.yahoo.com. MX
Answer
The response has no answer records.
Authority
yahoo.com. 600 SOA ns1.yahoo.com. hostmaster.yahoo-inc.com. 2026022102 3600 300 1814400 600
Additional
. 0 OPT ; payload 1272, xrcode 0, version 0, flags 0