yf4.a1.b.yahoo.net has no MX records.
Question and response
Question
dig yf4.a1.b.yahoo.net. MX
Answer
The response has no answer records.
Authority
a1.b.yahoo.net. 300 SOA yf1.yahoo.com. hostmaster.yahoo-inc.com. 1772076597 30 30 86400 300
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0