feat: expand mailbox evidence scanner

tests/fixtures/mailbox/complaint.eml

@@ -0,0 +1,10 @@
+From: Feedback Loop <fbl@example.net>
+To: abuse@example.com
+Subject: Spam complaint notification
+Date: Tue, 02 Jun 2026 10:04:00 +0000
+Message-ID: <complaint@example.net>
+Content-Type: text/plain; charset=utf-8
+
+Feedback loop abuse report.
+Final-Recipient: rfc822; complained@example.com
+This is a spam complaint notification for the original message.