This is the start of a new ongoing series for us that gives you the tips and tricks you need for successful DMARC deployment.
There are several safeguards put in place with SPF. One of these is a limitation of DNS lookups to help ensure that you do not have timeout issues. SPF will evaluate only 10 DNS mechanism lookups in an SPF record (mechanisms include: a, mx, ptr, exists, include, redirect) before a PermError is raised. Terms in included/redirected-to records count toward that total, which frequently causes records to error out when included from other records even though by themselves they stay within this limit. If you are seeing this error, you will need to remove some of your current mechanisms/lookups.