SPF supports up to 10 lookups. When implementing an maintaining an SPF record, it is important to limit the number of mechanisms and modifiers, that rely on DNS lookups, to no more than ten. Be careful and double check the number of lookups when using “include” mechanisms or a “redirect” modifier. Include mechanisms in particular are often not designed to be efficient and can put more than a dent in the 10 lookup limit.
Keep in mind that mechanisms such as “include”, “a”, “mx”, “ptr”, will require at least one lookup . Since mechanisms such as "ip4”, and “ip6” refer to raw IP addresses, they do not require DNS lookups and do not eat away at your 10 lookup limit and can be used to support a large number of hosts.
How can I better control the number of lookup in my SPF record?
- Prune unnecessary :include files from the SPF record. If you can use "ip4”, and “ip6 mechanisms, rather than “include”, “a”, “mx”, “ptr”, use them to save on lookups.
- Making use of different domains or subdomains for email purposes, so you can "spread" lookups across multiple domains might be an option.
There are many tools available that you can use to simplify the process of evaluating the number of lookups in your SPF record.Easy Dmarc has a great tool that you can try found here:. https://easydmarc.com/tools/spf