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

Results foroutboundmail.convio.net

Resolver

Location

A

Name	TTL	Value
outboundmail.convio.net	600	spf2.0/pra +ip4:216.235.195.0/24 +ip4:74.123.153.0/25 +ip4:74.123.154.128/25 +ip4:64.244.120.32/27 +ip4:64.244.122.192/27 +ip4:216.235.195.128/27 +ip4:216.235.195.160/27 +ip4:205.139.104.0/22 +include:sparkpostmail.com ~all
outboundmail.convio.net	600	v=spf1 +ip4:216.235.195.0/24 +ip4:74.123.153.0/25 +ip4:74.123.154.128/25 +ip4:64.244.120.32/27 +ip4:64.244.122.192/27 +ip4:216.235.195.128/27 +ip4:216.235.195.160/27 +ip4:205.139.104.0/22 +include:sparkpostmail.com ~all