spf-i.verisign.com has no MX records.
Question and response
Question
dig spf-i.verisign.com. MX
Answer
The response has no answer records.
Authority
verisign.com. 86400 SOA av1.nstld.com. mdnshelp.verisign.com. 1771806155 300 7200 1209600 86400
Additional
. 0 OPT ; payload 4096, xrcode 0, version 0, flags 0