exp.to.fortiva.net has no MX records.
Question and response
Question
dig exp.to.fortiva.net. MX
Answer
The response has no answer records.
Authority
fortiva.net. 600 SOA ns1.proofpoint.com. ops.proofpoint.com. 2017040655 10800 3600 604800 3600
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0