exists mechanism
The SPF mechanism that builds a domain name, usually from macros, and matches if that name has any A record, whatever the address.
What it means
exists:domain asks the DNS for an A record at the given name and matches if one comes back. On its own that is rarely useful; its power comes from SPF macros, which insert parts of the SMTP session into the name. exists:%{i}._spf.example.com, for instance, queries a name built from the connecting IP address, so the domain's DNS server can decide per address (or per sender) without the record listing them.
The query is always for an A record, even when the connection is IPv6, and the returned address is never compared with anything.
Each exists counts toward SPF's limit of ten DNS lookups.
Example
v=spf1 exists:%{i}._spf.example.com -allRelated terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 7208: Sender Policy Framework (SPF), section 5.7rfc-editor.org
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