f1.billingbooth.com has no MX records.
Question and response
Question
dig f1.billingbooth.com. MX
Answer
The response has no answer records.
Authority
billingbooth.com. 1800 SOA ivy.ns.cloudflare.com. dns.cloudflare.com. 2397142555 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0