mail3.slfs.com has no MX records.
Question and response
Question
dig mail3.slfs.com. MX
Answer
The response has no answer records.
Authority
slfs.com. 86400 SOA ns1.markmonitor.com. hostmaster.markmonitor.com. 2025120106 86400 3600 2592000 172800
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0