https://datatracker.ietf.org/doc/html/rfc7208#section-7

   When evaluating an SPF policy record, certain character sequences are
   intended to be replaced by parameters of the message or of the
   connection.  These character sequences are referred to as "macros".

1. history

7.1.  Formal Specification

   The ABNF description for a macro is as follows:


   Some special cases:

   o  A literal "%" is expressed by "%%".

   o  "%_" expands to a single " " space.

   o  "%-" expands to a URL-encoded space, viz., "%20".


7.2.  Macro Definitions

   The following macro letters are expanded in term arguments:

      s = <sender>
      l = local-part of <sender>
      o = domain of <sender>
      d = <domain>
      i = <ip>
      p = the validated domain name of <ip> (do not use)
      v = the string "in-addr" if <ip> is ipv4, or "ip6" if <ip> is ipv6
      h = HELO/EHLO domain


   <domain>, <sender>, and <ip> are defined in Section 4.1.

   The following macro letters are allowed only in "exp" text:

      c = SMTP client IP (easily readable format)
      r = domain name of host performing the check
      t = current timestamp

7.3.  Macro Processing Details

4.1.  Arguments

   The check_host() function takes these arguments:

   <ip>     - the IP address of the SMTP client that is emitting
              the mail, either IPv4 or IPv6.

   <domain> - the domain that provides the sought-after authorization
              information; initially, the domain portion of the
              "MAIL FROM" or "HELO" identity.

   <sender> - the "MAIL FROM" or "HELO" identity.

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