b23.verisigndns.com has no MX records.
Question and response
Question
dig b23.verisigndns.com. MX
Answer
The response has no answer records.
Authority
verisigndns.com. 86400 SOA av1.nstld.com. dnssupport.verisign-grs.com. 1771966980 28800 7200 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0