With the rest of the world starting to take a much bigger interest in preventing email spam, many of the old ways of setting up email domains are coming under increased scrutiny and are now presenting a risk to our ability to process emails for all CUs across our network. One of these is now the SPF (Sender Policy Framework) record on an email domain is setup.  The limit of 10 DNS lookups in an SPF record is an aspect of SPF design, primarily aimed at reducing the load on DNS servers and preventing abuse. Here's a more detailed explanation:

DNS Load Reduction: Each time an email is received, the recipient's mail server performs DNS lookups to verify the sender's SPF record. If SPF records allowed unlimited DNS lookups, it could significantly increase the load on DNS servers. This is especially true for high-volume email receivers like Gmail, Yahoo, or corporate email servers. By limiting the number of lookups, SPF helps to keep the DNS system efficient and responsive.

Preventing Abuse: Limiting the number of DNS lookups in SPF records also helps prevent certain types of abuse. For example, without a limit, an attacker could craft an SPF record that causes a mail server to perform an excessive number of DNS lookups, potentially leading to denial of service (DoS) attacks against either the mail server or the DNS system.

Simplifying Processing: A limit on DNS lookups simplifies the processing of SPF records. Mail servers can determine the sender's legitimacy more quickly and with fewer resources. This efficiency is crucial for maintaining fast email delivery times.

Encouraging Concise SPF Records: The limit encourages organizations to create concise and efficient SPF records. This often means carefully considering which mail servers are authorized to send emails on their behalf, leading to better email security practices.

Mechanisms Counting Towards the Limit: In SPF, several mechanisms and modifiers cause DNS lookups, such as include, a, mx, ptr, and exists. Each of these counts towards the limit of 10. However, mechanisms like all, ip4, and ip6 do not require DNS lookups and therefore do not count towards this limit.

Workarounds for Large Senders: Organizations with many sending sources (like third-party email services, multiple outbound mail servers, etc.) might struggle to stay within this limit. They often need to carefully plan their SPF records or use mechanisms like IP address ranges (ip4/ip6) instead of domain-based mechanisms (include, a, mx) to stay within the limit.

In summary, the 10 DNS lookup limit in SPF records is a balance between efficiency, security, and practicality, helping to ensure that the SPF protocol remains a viable and effective tool for preventing email spoofing and spam.