Outlook Auto Signature Add-in

An event-based Office.js add-in that automatically injects a remotely-managed HTML signature whenever a user opens a new compose window in Outlook. No task pane, no user interaction — it just works silently in the background.

Signature content is served from Cloudflare Pages, meaning it can be updated without touching the add-in or re-sideloading the manifest.

---

How It Works

1. User opens a new compose window in Outlook
2. Outlook fires the OnNewMessageCompose event
3. commands.js fetches signature.html from Cloudflare Pages
4. The signature is injected via setSignatureAsync() (falls back to body.setAsync() on older builds)
5. No UI, no button, no task pane

---

Repo Structure

├── manifest.xml       # Add-in manifest — sideloaded locally into Outlook
├── commands.js        # Event handler — hosted on Cloudflare Pages
├── signature.html     # HTML signature fragment — hosted on Cloudflare Pages
├── _headers           # Cloudflare Pages CORS header config
└── README.md

---

Requirements

• Outlook for Mac (2023 or later)
• Microsoft 365 account signed into Outlook
• A Cloudflare Pages account (free tier)
• A GitHub account

---

Deploy

1. Fork or clone this repo

git clone https://github.com/YOUR_USERNAME/YOUR_REPO_NAME.git
cd YOUR_REPO_NAME

2. Customise the signature

Edit signature.html with your real name, title, contact details, and logo URL.

3. Deploy to Cloudflare Pages

1. Log in to Cloudflare Dashboard → Workers & Pages → Create
2. Connect your GitHub account and select this repository
3. Set build configuration:

• Build command: (leave empty)
• Build output directory: /

1. Click Save and Deploy

Your files will be live at https://YOUR_PROJECT_NAME.pages.dev.

The _headers file automatically sets permissive CORS headers on commands.js and signature.html — no extra configuration needed.

4. Update the manifest

Open manifest.xml and replace every instance of YOUR_DOMAIN with your Cloudflare Pages URL:

<bt:Url id="Commands.Url" DefaultValue="https://YOUR_PROJECT_NAME.pages.dev/commands.js"/>

Also replace the <Id> field with a fresh GUID. On macOS:

uuidgen

Update commands.js too:

const SIGNATURE_URL = "https://YOUR_PROJECT_NAME.pages.dev/signature.html";

Push the changes — Cloudflare Pages will redeploy automatically.

5. Sideload in Outlook for Mac

1. Open Outlook
2. Insert → Add-ins → My Add-ins → ··· → Add a Custom Add-in → Add from File…
3. Select your local manifest.xml
4. Open a new compose window — the signature will appear automatically

---

Updating the Signature

Edit signature.html and push to main. Cloudflare Pages redeploys in seconds. No manifest changes, no re-sideloading required.

---

Troubleshooting

Signature doesn’t appear after sideloading

• Open commands.js and signature.html URLs directly in a browser to confirm they’re live and returning the expected content
• Check that all YOUR_DOMAIN placeholders in manifest.xml have been replaced
• Confirm the <Id> in manifest.xml is a valid GUID

Add-in loads but signature is blank

• Open browser DevTools on the Cloudflare Pages URL and check for CORS errors
• Verify the _headers file is present in the repo root and has deployed correctly

setSignatureAsync has no effect

• Outlook respects the user’s native signature settings — if a default signature is already configured in Outlook preferences, the API will not overwrite it. Remove the native signature under Outlook → Preferences → Signatures and try again.

Manifest rejected silently by Outlook

• Validate the XML is well-formed (no unclosed tags, no stray placeholder text)
• Ensure the Cloudflare Pages URLs are HTTPS

---

Architecture Notes

This is a proof of concept. The sideloaded manifest approach works for development and internal testing without requiring Microsoft 365 admin access or AppSource publishing. For a production rollout, the manifest would be deployed via the Microsoft 365 admin centre, making the add-in available to all users in an organisation automatically.

---

License

MIT