feat(email): forward non-feed mail to FALLBACK_FORWARD_ADDRESS

Lets you point a domain's catch-all at the worker without losing personal
mail: inbound mail that isn't a feed (invalid_address / feed_not_found) is
forwarded to an optional verified destination instead of being dropped.
Expired feeds and blocked senders are still dropped so newsletters never
leak to the fallback inbox. Unset env keeps the original drop-and-log path.

Co-Authored-By: Claude Opus 4.7 <[email protected]>

src/infrastructure/cloudflare-email.ts

@@ -1,6 +1,10 @@
 import PostalMime from "postal-mime";
 import { Env } from "../types";
-import { processEmail, RawAttachment } from "../application/email-processor";
+import {
+  processEmail,
+  RawAttachment,
+  IngestRejectionReason,
+} from "../application/email-processor";
 import { normalizeCid } from "../infrastructure/html-processor";
 import { logger } from "./logger";
 
@@ -51,8 +55,36 @@ export async function handleCloudflareEmail(
         to: message.to,
         reason: result.reason,
       });
+      await maybeForwardFallback(message, env, result.reason);
     }
   } catch (error) {
     console.error("Error processing Cloudflare email:", error);
   }
 }
+
+// Reasons safe to forward to the catch-all fallback: the mail was never a feed's
+// (wrong address shape, or no such feed). Expired feeds and blocked senders are
+// dropped so a real newsletter never leaks into the fallback inbox.
+const FORWARDABLE_REASONS = new Set<IngestRejectionReason>([
+  "invalid_address",
+  "feed_not_found",
+]);
+
+async function maybeForwardFallback(
+  message: ForwardableEmailMessage,
+  env: Env,
+  reason: IngestRejectionReason,
+): Promise<void> {
+  const fallback = env.FALLBACK_FORWARD_ADDRESS;
+  if (!fallback || !FORWARDABLE_REASONS.has(reason)) return;
+
+  try {
+    await message.forward(fallback);
+  } catch (error) {
+    logger.warn("Fallback forward failed", {
+      to: message.to,
+      fallback,
+      error: error instanceof Error ? error.message : String(error),
+    });
+  }
+}