exp modifier
The SPF modifier exp=domain, which points to a TXT record holding the explanation text a receiver may return when a message fails SPF.
What it means
When a check ends in fail, a receiver can include an explanation in its rejection. exp=explain.example.com names a TXT record whose text, after macro expansion, becomes that explanation, typically a sentence and a link to the domain's mail policy. RFC 7208 asks receivers to make clear the text comes from the sender's domain.
The lookup happens only after a fail, so exp does not count toward the limit of ten DNS lookups. If the explanation record is missing, malformed or returns more than one TXT record, the receiver carries on as if there were no exp.
Few domains publish one and many receivers ignore it, so its absence is never a fault.
Example
v=spf1 mx -all exp=explain._spf.%{d}Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 7208: Sender Policy Framework (SPF), section 6.2rfc-editor.org
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