mta3.serverlet.com has no MX records.
Question and response
Question
dig mta3.serverlet.com. MX
Answer
The response has no answer records.
Authority
serverlet.com. 3600 SOA ns1.serverlet.com. info.serverlet.com. 2026022394 10000 7200 604800 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0