Docs: add note on spf lookup type, in experimental-spec

diff --git a/doc/doc-txt/experimental-spec.txt b/doc/doc-txt/experimental-spec.txt
index f0c6d95ba70699ea71f5c1fe508c780b9de5ca39..317f40101c74f46535d47305290013cde7fb1e25 100644 (file)
--- a/doc/doc-txt/experimental-spec.txt
+++ b/doc/doc-txt/experimental-spec.txt
@@ -448,6 +448,17 @@ spf_guess = v=spf1 a/16 mx/16 ptr ?all
 would relax host matching rules to a broader network range.
 
 
+A lookup expansion is also available. It takes an email
+address as the key and an IP address as the database:
+
+  $lookup (username@domain} spf {ip.ip.ip.ip}}
+
+The lookup will return the same result strings as they can appear in
+$spf_result (pass,fail,softfail,neutral,none,err_perm,err_temp).
+Currently, only IPv4 addresses are supported.
+
+
+
 SRS (Sender Rewriting Scheme) Support
 --------------------------------------------------------------