SPF Records – DNS lookups must be under 10 per SPF check
Section 10.1 of the SPF RFC:
SPF implementations MUST limit the number of mechanisms and modifiers that do DNS lookups to at most 10 per SPF check, including any lookups caused by the use of the “include” mechanism or the “redirect” modifier. If this number is exceeded during a check, a PermError MUST be returned. The “include”, “a”, “mx”, “ptr”, and “exists” mechanisms as well as the “redirect” modifier do count against this limit. The “all”, “ip4″, and “ip6″ mechanisms do not require DNS lookups and therefore do not count against this limit. The “exp” modifier does not count against this limit because the DNS lookup to fetch the explanation string occurs after the SPF record has been evaluated.
