diff --git a/instrumentation.ts b/instrumentation.ts
index dcc6b96..93b72ed 100644
--- a/instrumentation.ts
+++ b/instrumentation.ts
@@ -10,4 +10,11 @@ export async function register() {
   await ensureSsoCacheLoaded().catch((error) => {
     console.error("Failed to warm SSO cache during instrumentation", error);
   });
+
+  // Seal any plaintext Resend API keys carried over from before AES-GCM at-rest
+  // encryption shipped. Idempotent — rows already sealed are skipped.
+  const { backfillSealedResendKeys } = await import("./lib/auth/secret-backfill");
+  await backfillSealedResendKeys().catch((error) => {
+    console.error("Failed to backfill sealed resend keys during instrumentation", error);
+  });
 }
diff --git a/lib/auth/secret-backfill.ts b/lib/auth/secret-backfill.ts
new file mode 100644
index 0000000..6fa90f5
--- /dev/null
+++ b/lib/auth/secret-backfill.ts
@@ -0,0 +1,61 @@
+import { eq, sql } from "drizzle-orm";
+
+import { sealTotpSecret } from "@/lib/auth/totp";
+import { db } from "@/lib/db/client";
+import { organisations } from "@/lib/db/schema";
+
+/**
+ * Resend API key prefix. New keys (`re_…`) are short enough that an
+ * accidentally-plaintext value still fits inside the `text` column that
+ * also stores AES-GCM ciphertext (base64). The pattern below matches the
+ * complete plaintext shape so we never mistake a sealed value (random
+ * base64) for a plaintext key.
+ */
+const RESEND_PLAINTEXT_PATTERN = /^re_[A-Za-z0-9_-]+$/;
+
+export type ResendKeyBackfillResult = {
+  scanned: number;
+  sealed: number;
+  errors: string[];
+};
+
+/**
+ * Find any rows whose `resend_api_key_encrypted` column still holds a
+ * plaintext Resend key (legacy data from before the AES-GCM wrapper
+ * landed) and seal them at rest. Safe to run on every boot — idempotent
+ * because rows that no longer match the plaintext pattern are skipped.
+ */
+export async function backfillSealedResendKeys(): Promise<ResendKeyBackfillResult> {
+  const result: ResendKeyBackfillResult = { scanned: 0, sealed: 0, errors: [] };
+
+  const candidates = await db
+    .select({
+      id: organisations.id,
+      resendApiKeyEncrypted: organisations.resendApiKeyEncrypted,
+    })
+    .from(organisations)
+    .where(sql`${organisations.resendApiKeyEncrypted} ~ '^re_[A-Za-z0-9_-]+$'`);
+
+  result.scanned = candidates.length;
+
+  for (const row of candidates) {
+    if (!row.resendApiKeyEncrypted || !RESEND_PLAINTEXT_PATTERN.test(row.resendApiKeyEncrypted)) {
+      continue;
+    }
+    try {
+      const sealed = sealTotpSecret(row.resendApiKeyEncrypted);
+      await db
+        .update(organisations)
+        .set({ resendApiKeyEncrypted: sealed })
+        .where(eq(organisations.id, row.id));
+      result.sealed += 1;
+      console.log(`[secret-backfill] sealed plaintext resend key for org=${row.id}`);
+    } catch (error) {
+      const message = error instanceof Error ? error.message : String(error);
+      result.errors.push(`${row.id}: ${message}`);
+      console.error(`[secret-backfill] failed to seal resend key org=${row.id}: ${message}`);
+    }
+  }
+
+  return result;
+}
diff --git a/lib/email/campaign-sender.ts b/lib/email/campaign-sender.ts
index 63d4a9b..70f4dd2 100644
--- a/lib/email/campaign-sender.ts
+++ b/lib/email/campaign-sender.ts
@@ -296,12 +296,11 @@ export type OrganisationTransportConfig = {
 
 function decryptIfSet(sealed: string | null): string | null {
   if (!sealed) return null;
-  try {
-    return openTotpSecret(sealed);
-  } catch (error) {
-    if (/^re_[A-Za-z0-9_-]+$/.test(sealed)) return sealed;
-    throw error;
-  }
+  // No plaintext fallback: if `openTotpSecret` throws, the AES-GCM auth tag
+  // failed or the column is not sealed. Either way we fail closed rather than
+  // hand the caller a credential we cannot vouch for. Plaintext rows are
+  // sealed on boot by `backfillSealedResendKeys` (see instrumentation.ts).
+  return openTotpSecret(sealed);
 }
 
 export function getTransportForOrganisation(org: OrganisationTransportConfig): CampaignTransport {
diff --git a/scripts/capture-readme-screenshots.ts b/scripts/capture-readme-screenshots.ts
index 8d5a56d..a17858f 100644
--- a/scripts/capture-readme-screenshots.ts
+++ b/scripts/capture-readme-screenshots.ts
@@ -5,6 +5,7 @@ import { dirname, resolve } from "node:path";
 import { spawn, type ChildProcess } from "node:child_process";
 import { randomUUID } from "node:crypto";
 
+import { sealTotpSecret } from "@/lib/auth/totp";
 import { db, sql as dbSql } from "@/lib/db/client";
 import {
   campaignTargets,
@@ -209,7 +210,7 @@ async function seedProductData(): Promise<SeedResult> {
   await db
     .update(organisations)
     .set({
-      resendApiKeyEncrypted: "re_readme_capture_fake_key",
+      resendApiKeyEncrypted: sealTotpSecret("re_readme_capture_fake_key"),
       senderFromAddress: "Collie Training <training@example.test>",
       updatedAt: new Date(),
     })