mail3.sendersrv.com has no MX records.
Question and response
Question
dig mail3.sendersrv.com. MX
Answer
The response has no answer records.
Authority
sendersrv.com. 1800 SOA edna.ns.cloudflare.com. dns.cloudflare.com. 2398065559 10000 2400 604800 1800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0