Set list-id header for notification mails

modules/joex/src/main/resources/reference.conf

@@ -31,6 +31,19 @@ docspell.joex {
     password = ""
   }
 
+  send-mail {
+    # This is used as the List-Id e-mail header when mails are sent
+    # from docspell to its users (example: for notification mails). It
+    # is not used when sending to external recipients. If it is empty,
+    # no such header is added. Using this header is often useful when
+    # filtering mails.
+    #
+    # It should be a string in angle brackets. See
+    # https://tools.ietf.org/html/rfc2919 for a formal specification
+    # of this header.
+    list-id = ""
+  }
+
   # Configuration for the job scheduler.
   scheduler {