ums-dns-salesforce.com has no MX records.
Question and response
Question
dig ums-dns-salesforce.com. MX
Answer
The response has no answer records.
Authority
ums-dns-salesforce.com. 3600 SOA ns01.ums-dns-salesforce.com. hostmaster.nsone.net. 1710355118 43200 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0