ns01.squarespacedns.com has no MX records.
Question and response
Question
dig ns01.squarespacedns.com. MX
Answer
The response has no answer records.
Authority
squarespacedns.com. 3600 SOA dns1.p06.nsone.net. hostmaster.nsone.net. 1634829256 43200 7200 1209600 3600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0