While directly including IP addresses in SPF records is permissible and doesn't consume DNS lookup slots, it's generally advised against for larger setups. The 'include:' mechanism is favored for its scalability, ease of maintenance (especially with third-party services), and avoidance of exceeding the SPF's 10 DNS lookup limit. Techniques like SPF flattening and CIDR notation can also help manage IP addresses effectively. Understanding the intent behind authorizing specific IPs, and ensuring ESPs use dedicated subdomains for SPF are also vital. For long term maintainability consider moving to using include or domains that dynamically update IP addresses, rather than static IP addresses.