mail-sf1.voip.ms has no MX records.
Question and response
Question
dig mail-sf1.voip.ms. MX
Answer
The response has no answer records.
Authority
voip.ms. 1800 SOA chan.ns.cloudflare.com. dns.cloudflare.com. 2398526707 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0