diff --git a/dkimpy-milter.conf.5 b/dkimpy-milter.conf.5
index 8c64767..99611d3 100644
--- a/dkimpy-milter.conf.5
+++ b/dkimpy-milter.conf.5
@@ -213,6 +213,12 @@ The value may include two different canonicalizations separated by a
 slash ("/") character, in which case the first will be applied to the
 header and the second to the body.
 
+.TP
+.I DiagnosticDirectory (string)
+Directory into which to write diagnostic reports when message verification
+fails on a message bearing a "z=" tag.  If not set (the default), these files
+are not generated.
+
 .TP
 .I Domain (dataset)
 A set of domains whose mail should be signed by this filter.  Mail from other