It can be anything! An apex, subdomain, or even a URL.

Results forspf.smtp2go.com

Resolver

Location

TXT

Name	TTL	Value
spf.smtp2go.com	14400	v=spf1 ip4:207.58.147.64/28 ip4:216.22.15.224/27 ip4:43.228.184.0/22 ip4:103.47.204.0/22 ip4:103.2.140.0/22 ip4:203.31.36.0/22 ip4:170.10.68.0/22 ip4:158.120.80.0/21 ip4:66.235.120.0/21 ip4:194.195.251.175 ~all