Tag spam subject

mail/exim4/system_filter

@@ -8,3 +8,14 @@ if first_delivery then
     headers add $acl_m3
   endif
 endif
+
+# Prefix the subject of mail identified as spam if it's not already prefixed
+if $h_X-Spam-Status: contains "spam" then
+  if $header_subject: does not contain "[spam]" then 
+    # replace existing subject header as 'Original-Subject'
+    headers add "Original-Subject: $h_subject"
+    headers remove "Subject"
+    # add new subject header with prefix
+    headers add "Subject: [spam] $h_original-subject"
+  endif
+endif