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What is the policy that is applied when "all" is missing from an SPF record? For example as in v=spf1 a ip4:198.51.100.15.

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As specified by the RFC 7208, the default behavior is to return a "neutral" result, meaning as if ?all was specified.

The following v=spf1 a ip4:198.51.100.15 and therefore equivalent to v=spf1 a ip4:198.51.100.15 ?all.

If none of the mechanisms match and there is no "redirect" modifier, then the check_host() returns a result of "neutral", just as if "?all" were specified as the last directive. If there is a "redirect" modifier, check_host() proceeds as defined in Section 6.1.

It is better to use either a "redirect" modifier or an "all" mechanism to explicitly terminate processing. Although there is an implicit "?all" at the end of every record that is not explicitly terminated, it aids debugging efforts when it is explicitly provided.

For example:

v=spf1 +mx -all

or

v=spf1 +mx redirect=_spf.example.com
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