SPF Record Syntax: Mechanisms, Qualifiers, Modifiers, and Macros

SPF (Sender Policy Framework) records are single DNS TXT records starting with "v=spf1" that contain mechanisms, qualifiers, and modifiers to authorize which servers can send email for a domain. The record is evaluated left to right until the first match, with common mechanisms including include, ip4, ip6, and all to specify authorized senders and reject unauthorized ones. SPF syntax is strictly defined in RFC 7208, and any syntax error invalidates the entire record, potentially failing authentication for all messages from a domain.