diff --git a/src/lib/postguard.ts b/src/lib/postguard.ts
index 1c8f2fc..a2d4786 100644
--- a/src/lib/postguard.ts
+++ b/src/lib/postguard.ts
@@ -17,6 +17,11 @@ export const retryStatus = writable<RetryEvent | null>(null)
 export const pg = new PostGuard({
     pkgUrl: PKG_URL,
     cryptifyUrl: FILEHOST_URL,
+    // Identifies traffic from this site in cryptify's per-channel
+    // upload metrics. Without this, cryptify falls back to matching the
+    // browser Origin header, which collides with the Outlook add-in
+    // (also served from a *.postguard.eu host).
+    headers: { 'X-Cryptify-Source': 'website' },
     ...(CHUNK_SIZE !== undefined && { uploadChunkSize: CHUNK_SIZE }),
     retry: {
         // Defaults (5 attempts, 500ms initial) exhaust the retry budget in