diff --git a/src/src/pdkim/pdkim.h b/src/src/pdkim/pdkim.h
index 0c9d46d011..5889eb2ba2 100644
--- a/src/src/pdkim/pdkim.h
+++ b/src/src/pdkim/pdkim.h
@@ -26,14 +26,14 @@
 #include "../blob.h"
 #include "../hash.h"
 
-#define PDKIM_DEFAULT_SIGN_HEADERS "From:Sender:Reply-To:Subject:Date:"\
+#define PDKIM_DEFAULT_SIGN_HEADERS "From:=Sender:Reply-To:Subject:Date:"\
                              "Message-ID:To:Cc:MIME-Version:Content-Type:"\
                              "Content-Transfer-Encoding:Content-ID:"\
-                             "Content-Description:Resent-Date:Resent-From:"\
-                             "Resent-Sender:Resent-To:Resent-Cc:"\
-                             "Resent-Message-ID:In-Reply-To:References:"\
-                             "List-Id:List-Help:List-Unsubscribe:"\
-                             "List-Subscribe:List-Post:List-Owner:List-Archive"
+                             "Content-Description:=Resent-Date:=Resent-From:"\
+                             "=Resent-Sender:=Resent-To:=Resent-Cc:"\
+                             "=Resent-Message-ID:In-Reply-To:References:"\
+                             "=List-Id:=List-Help:=List-Unsubscribe:"\
+                             "=List-Subscribe:=List-Post:=List-Owner:=List-Archive"
 
 /* -------------------------------------------------------------------------- */
 /* Length of the preallocated buffer for the "answer" from the dns/txt