mx.theresslergroup.com has no MX records.
Question and response
Question
dig mx.theresslergroup.com. MX
Answer
The response has no answer records.
Authority
theresslergroup.com. 3600 SOA dns.theresslergroup.com. hostmaster.theresslergroup.com. 1605823066 43200 3600 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0