b.dns.tw has no MX records.
Question and response
Question
dig b.dns.tw. MX
Answer
The response has no answer records.
Authority
tw. 900 SOA a.dns.tw. snw.twnic.net.tw. 2010989539 3600 900 1296000 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0