fix: do not modify DKIM signature header keys such as X-Report-Abuse-…

…To if signed in DKIM signature header value

helpers/on-data-mx.js

@@ -1427,7 +1427,7 @@ async function onDataMX(raw, session, headers, body) {
   //
 
   // add X-* headers (e.g. version + report-to)
-  await updateHeaders(headers);
+  await updateHeaders(headers, session);
 
   // additional headers to add specifically for MX
   // (this also does a friendly-from rewrite if necessary)

helpers/update-headers.js

@@ -5,19 +5,36 @@
 
 const config = require('#config');
 
-function updateHeaders(headers) {
-  for (const key of [
-    'x-report-abuse-to',
-    'x-report-abuse',
-    'x-complaints-to',
-    'x-forwardemail-version'
-  ]) {
-    headers.remove(key);
+//
+// NOTE: we pass `headers` here because we don't want to update header values
+//       for those that were signed with someone's DKIM signature
+//       (e.g. algolia.com signs via mailjet with X-Report-Abuse-To as a key)
+//       and if we modify it then the header chain will break of course
+//
+function updateHeaders(headers, session) {
+  const keys = new Set();
+
+  if (
+    Array.isArray(session?.dkim?.results) &&
+    session.dkim.results.length > 0
+  ) {
+    for (const result of session.dkim.results) {
+      if (typeof result?.signingHeaders?.keys !== 'string') continue;
+      for (const key of result.signingHeaders.keys.split(':')) {
+        keys.add(key.toLowerCase().trim());
+      }
+    }
   }
 
-  headers.add('X-Report-Abuse-To', config.abuseEmail, headers.lines.length);
-  headers.add('X-Report-Abuse', config.abuseEmail, headers.lines.length);
-  headers.add('X-Complaints-To', config.abuseEmail, headers.lines.length);
+  if (!keys.has('x-report-abuse-to'))
+    headers.add('X-Report-Abuse-To', config.abuseEmail, headers.lines.length);
+
+  if (!keys.has('x-report-abuse'))
+    headers.add('X-Report-Abuse', config.abuseEmail, headers.lines.length);
+
+  if (!keys.has('x-complaints-to'))
+    headers.add('X-Complaints-To', config.abuseEmail, headers.lines.length);
+
   headers.add(
     'X-ForwardEmail-Version',
     config.pkg.version,