Guardcell for Outlook

Asks before it leaves.

Checks run inside Outlook on each sender's device. Nothing in the email is sent anywhere to be checked.

Make your manifest

Your mailbox's own domain is always inside. Add your other domains if you need them.

Serve the contents of dist at this HTTPS address, including its subpath.

Technical mapping and client support

Outlook versions from roughly 2022 on can show Send Anyway for Ask outcomes; older ones hold them when either setting is Hold. An exposed recipient list or an attached picture it couldn't read always asks where supported. Newer versions offer Review to open the pane; older ones may show Review on the button that cancels sending, or give ribbon instructions.

The on-device model runs in Outlook on the web, the new Outlook for Windows and Outlook for Mac; classic Outlook for Windows uses the rules only.

Exact matches only. Subdomains must be listed separately.

Real partners whose domain looks like yours, so it isn't flagged as a lookalike. Email to them is still checked like any other outside email.

Version and add-in Id

Keep the same Id when updating this installation. Use a higher version.

Install for your organization

  1. Choose your host. Use the public release address above, or host the built dist/ folder yourself over HTTPS. For your own host, publish the files before using its manifest.
  2. Upload the manifest. In the Microsoft 365 admin center, go to Settings > Integrated apps > Upload custom apps. Choose the add-in manifest and select manifest.xml.
  3. Assign and test. Assign users or groups, finish deployment, then test a synthetic email and meeting invitation in Outlook. Start with both settings on Ask and allow time for Microsoft 365 to apply the installation.
  4. If either setting is Hold, set three Outlook policies. OnSendAddinsEnabled for web and new Outlook, OnSendAddinsWaitForLoad on Mac, and the classic Outlook Group Policy Running Outlook for Simple MAPI Mail Sending. Without them, some sends skip the check. These are relevant to holding; when failures are set to Ask, Outlook still offers Send Anyway if the add-in cannot load. Details are in the README.

Outlook on the web, new and classic Outlook for Windows, and Outlook for Mac on Exchange Online. Mobile Outlook and mail sent by other apps are outside this check.