o3.em.squarespace.com has no MX records.
Question and response
Question
dig o3.em.squarespace.com. MX
Answer
The response has no answer records.
Authority
squarespace.com. 900 SOA dns1.p06.nsone.net. dns-admin.squarespace.com. 1663787404 3600 600 604800 900
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0