cname.vercel-dns.com has no MX records.
Question and response
Question
dig cname.vercel-dns.com. MX
Answer
The response has no answer records.
Authority
cname.vercel-dns.com. 900 SOA ns1.vercel-dns-3.com. awsdns-hostmaster.amazon.com. 1 3600 900 1209600 900
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0