The limit of 10 DNS lookups refers to a restriction found in some implementations of the SPF (Sender Policy Framework) mechanism. This limitation means that the mail server performing SPF record verification will not exceed a total of 10 DNS queries when checking a single email message to determine if the sender is authorized to send messages from a particular domain.

If the number of DNS queries exceeds the limit of 10, the mail server may take various actions depending on its configuration. This could include applying a "SOFTFAIL" policy, disregarding additional SPF mechanisms, or even rejecting the message as suspicious.

However, not all mail servers adhere to the exact same limit, so it may vary depending on the configuration of the specific mail server.

It involves tallying each DNS query operation performed by the mail server to determine if a given sender is authorized to send messages from a particular IP address or domain.

Below is the general procedure for counting lookups in an SPF record:

Basic SPF record: Each SPF record has initial 1 lookup cost.This means that initially, without any additional mechanisms, we already have 1 lookup.

SPF mechanisms: Each SPF mechanism (e.g., a, mx, include, ip4, ip6) requires an additional DNS lookup. For example, if an SPF record contains the mechanism include:example.com, this means that the server must conduct an additional lookup to check the SPF record for the example.com domain.

SPF modifiers: Some SPF modifiers, such as redirect, may cause the server to process additional SPF records from another domain, thereby increasing the number of lookups.

The number of lookups in an SPF record is the sum of all DNS query operations conducted by the mail server during the verification of the sender of the email message.