sb.registrar-servers.com has no MX records.
Question and response
Question
dig sb.registrar-servers.com. MX
Answer
The response has no answer records.
Authority
sb.registrar-servers.com. 3600 SOA edns1.registrar-servers.com. support.namecheap.com. 1757325359 3600 1801 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0