_spf.schwab.com has no NS records.
NS records for this zone are defined at schwab.com.
NS records for this zone are defined at schwab.com.
Question and response
Question
dig _spf.schwab.com. NS
Answer
The response has no answer records.
Authority
schwab.com. 600 SOA d33pf2cdc.schwab.com. dnsadmin.schwab.com. 2023022029 3600 900 604800 600
Additional
. 0 OPT ; payload 1232, xrcode 0, version 0, flags 0