feat: expand mailbox evidence scanner

tests/fixtures/mailbox/final_failure.eml

@@ -0,0 +1,12 @@
+From: Mail Delivery Subsystem <mailer-daemon@example.net>
+To: sender@example.com
+Subject: Final failure
+Date: Tue, 02 Jun 2026 10:06:00 +0000
+Message-ID: <final-failure@example.net>
+Content-Type: text/plain; charset=utf-8
+
+Delivery Status Notification.
+Final-Recipient: rfc822; expired@example.com
+Action: failed
+Status: 5.4.7
+Diagnostic-Code: smtp; Could not deliver after retry period. Final failure, giving up.