SPF "a" mechanism
The SPF mechanism that matches when the sending IP address is one of the A or AAAA addresses of the current domain or of a named host.
What it means
a on its own tests the IP addresses of the domain being checked; a:mail.example.com tests another host's addresses. The receiver looks up A records for an IPv4 connection and AAAA records for an IPv6 connection, so the one mechanism covers both. An optional prefix length widens the match to a network, as in a/24.
It suits small setups where the website host also sends mail, but it ties mail authorization to whatever the A record points at. When the website moves, a mail server at the old address stops matching and the new web host becomes an authorized sender. ip4 and ip6 state the addresses directly and need no lookup.
Each a mechanism counts toward SPF's limit of ten DNS lookups.
Example
v=spf1 a mx -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 5.3rfc-editor.org
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