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

Results formx.mailii.org

Resolver

Location

TXT

Name	TTL	Value
mx.mailii.org	300	v=spf1 ip4:78.129.155.110 ip4:62.60.202.245 ip4:107.158.62.200 ip4:78.129.155.173 ip4:87.248.129.51 ip4:185.208.79.100 ip4:62.60.204.67 ip4:185.208.79.18 ip4:46.246.29.237 ip4:185.73.201.179 ip4:87.248.129.253 ip4:185.173.104.5 ip4:109.169.2.207 ip4:109.1 69.88.245 ip4:45.136.5.243 ip4:158.255.77.0/24 ip4:91.132.166.0/24 -all