rb.slc.efscloud.net has no MX records.
Question and response
Question
dig rb.slc.efscloud.net. MX
Answer
The response has no answer records.
Authority
rb.slc.efscloud.net. 600 SOA ns1.efscloud.net. root.efscloud.net. 1652275552 7200 900 3600000 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0