You may wonder what the "Specify domain" box (shown below) is under the SPF Part and when to use it.

Let's say that your top level domain eg. example.com is used by a service such as Vuture. Vuture uses this top level domain in the From header to send the emails but Vuture requires a subdomain to be used for the Return-Path header of the emails (this is the header that SPF validation is done against). So under example.com you create a subdomain called vx.example.com. The SPF record that authorizes Vuture to send emails needs to be created under vx.example.com and not under example.com.

In the above case when Vuture is marked as an Asset for the domain example.com, in the SPF part field you need to use the "Specify domain" box and type in vx.example.com as shown below because that is where the SPF record exists.

When you save your changes the OnDMARC platform will use the domain specified under the "Specify domain" box to look for the SPF Part and will turn green if it is correct.