The Mailgun Inspect Chrome extension lets you test the email you're building right where you're building it. Instead of copying HTML out of your email editor and pasting it somewhere else, you click the extension, pick your clients and checks, and get previews and issue reports in a side panel next to your work — with flagged issues highlighted directly in the html.

This guide walks you through installing the extension, connecting it to your Mailgun account, and running your first preview.

Before you begin

You'll need all of the following:

Which features your plan includes

Previews work on every Inspect plan. The content checks are plan-dependent:

If a check isn't included in your plan, the extension still runs and shows you previews — the corresponding tab will simply tell you the feature isn't enabled on your account. To add a check, upgrade from Plan & Billing in the Mailgun control panel.

Step 1: Install the extension

1. Open the Mailgun Inspect listing in the Chrome Web Store. You can also search the Web Store for Mailgun Inspect. 
2. Click Add to Chrome.
3. Review permissions and click Add extension to confirm.

Pin the Extension

Chrome hides new extensions behind the puzzle-piece icon by default. Pinning puts Mailgun Inspect one click away, which matters because opening the side panel is how you use it.

1. Click the puzzle-piece icon in the Chrome toolbar.
2. Find Mailgun Inspect  and click the pin icon next to it.

Remember: If you are going to use it immediately, refresh any tabs you already had open – the extension can’t reach into pages that were already loaded when you installed the extension.

Step 2: Get your Mailgun API key

The extension authenticates to the Mailgun Inspect API with your own API key. If you already have an account-level key saved somewhere safe, skip to Step 3.

1. Log into your Mailgun Account. 
2. Click your Profile Menu in the top-right corner, then select API Security.
3. Click Add new key.
4. Enter a description you'll recognize later — for example, “Chrome: Inspect extension”.
5. Set the role to Admin, then click Create key.

Copy the key immediately. Mailgun displays the key secret only once, at creation. If you lose it, you can't retrieve it — you'll have to create a new one. Store it in your password manager.

There's a second reason to keep it handy, covered in the next step: unless you tell the extension to remember your key, you'll be pasting it in again each browser session.

Two things to watch for:

• Use an account API key, not a domain sending key. Domain sending keys are restricted to sending messages and won't authenticate against the Inspect API.
• The key needs the Admin role. 

Step 3: Connect the extension to your account

Save your API key

1. Click the Mailgun Inspect icon in your Chrome toolbar. The side panel opens.
2. Click Open Settings.
3. Paste your API key.
4. Decide whether to have the extension remember your key for up to 30 days. If you skip this, you'll paste the key in again each time you open the extension in a new browser session.
5. Choose your region — US or EU.
6. Select which QA checks you want by default. We recommend selecting all of them. If your subscription doesn’t have access to these checks, you can still check them – they just won’t be included in your tests.

Selecting region

Mailgun operates separate US and EU infrastructure, and your API key only works in the region where your account lives.

The default is US, so change it if your account is in the EU region.

How your API key is stored

You have two options here:

By default, your API key is held in the browser's session storage only — nothing is written to disk, and the key is cleared when you close your browser. The next time you open the extension, you paste it in again.

If you'd rather not re-enter it every session, choose the remember option when you connect. The extension then encrypts the key and stores it in your browser profile for up to 30 days, after which you'll be asked for it again.

Either way, keep the key in your password manager rather than trying to memorize it — even with the remember option, you'll need it at least once a month.

Step 4: Generate your first preview

Navigate to the email you want to test inside of Chrome. Then click the Mailgun Inspect extension to open the side panel.

Editor Compatibility

The extension will let you know whether it is compatible with the current editor on the top of the side panel. There are 3 options:

1. ESP Ready: When the current page is fully compatible, the message will identify the ESP and say ready, with the color set to green. This means you are good to run the test.
2. ESP Detected: If the extension can identify the ESP, but you are not yet on a page that it can pull html from, it will say the name of the ESP and “detected”. It will attempt to provide guidance on getting to the right page. The color here is yellow. 
3. Will capture full page HTML: This means it cannot identify the ESP or editor yet, so if you run a test, it will pull in the HTML from the full page. In general, this will not result in a good test, unless you are hosting the html on a page with nothing else. The color here is grey.

Some things to know:

• You can always run a test, no matter what page you are on, but in general, it is better to wait until you get the green message.
• The page status should update when you move to a new page, but there is a refresh button in case it does not. If you are on your editor, and it isn’t green, press refresh to see if that works.
• Different ESPs have different rules. First try to pull in the editor page, where you are typing the HTML. If that doesn’t get the green light, click “Preview” (or whatever your ESP equivalent it). This is usually the feature that shows you the email, and allows you to send a preview test. Often the extension will work when you open this page/frame.
• If the extension isn’t picking up anything on your editor, reach out to support and let us know. We can work to add that editor to the list of compatible editors for the extension. 

The current list of compatible editors/ESPs:

Step 5: Select your clients and checks, then run

In the side panel, choose which email clients to render against and which checks to run, then click Run Preview.

Nothing is sent anywhere until you click Run Preview. That click is the moment the extension reads the HTML of the email you're editing and submits it to your Mailgun account.

The extension sends the HTML of the active editor to the Mailgun Inspect API and then polls for results. Renderings arrive progressively — real email client screenshots take time to produce, so expect to wait rather than seeing everything at once.

How client selection affects your usage

Before you select every client in the catalogue:

One preview = one client rendering. If you run a test against 100 clients, that consumes 100 previews from your monthly allotment — not one. A Basic plan includes 1,000 previews per month, and Premium includes 2,000. Beyond your allotment, subscription plans are charged $0.15 per additional preview; contract plans use the overage rate in the contract.

If you are concerned about over usage: pick the clients your audience actually uses rather than selecting everything. A focused set of 30 to 50 clients covering your real open data will catch the most important rendering problems while making your allotment last 2-3 times longer

Step 6: Review your results

Results come back from your Mailgun account into the side panel, organized into three areas.

Previews is the first section. It shows how your email renders across the clients you selected, including desktop, web, mobile, and dark-mode variants.

QA Checks is a separate tab holding the four automated checks:

• Accessibility — WCAG-relevant issues, each with the offending HTML snippet and selector.
• Link Validation — broken links, missing URL schemes, and redirect issues.
• Image Validation — missing images and oversized assets.
• Code Analysis — HTML and CSS support reporting across email clients.

Spam Testing is its own section, and unlike the QA Checks it does not run on its own — you have to activate it. If you want spam testing results, go to the bottom of the QA Checks section, under Spam Testing, enter in a subject line and from name, and click Run Spam Test. This will take a few minutes to get results back. 

Once you make updates to your code, you can easily run another preview by clicking New Test, and you can select only the problematic previews the second time around to save usage. 

And remember, you can see these tests inside of Mailgun Inspect as well by clicking View full results in Mailgun at the top of the side panel.

Troubleshooting

The side panel won't open, or the floating beacon doesn't appear. Reload the page. The extension can't inject into tabs that were already open when it was installed or updated, and a reload is the fix. If the beacon still doesn't show, confirm the extension is enabled at chrome://extensions.

The extension can't find your email HTML. Reload the page and try again. If it still can't read your editor, fall back to the generic path: open the email's HTML in its own browser tab and run the preview from there.

It's asking for your API key again. That's expected unless you chose the remember option — by default the key lives in session storage and is cleared when the browser closes. If you did choose to remember it, the stored key expires after 30 days and the extension asks again. 

Authentication or "unauthorized" errors. Work through these in order: confirm the region you selected matches the region your Mailgun account lives in; confirm you used an account API key rather than a domain sending key; confirm your API key has admin permissions; then confirm the key hasn't been disabled or deleted in API Security.

A section says the feature isn't enabled. That check isn't included in your current Inspect plan. 

You're using previews faster than expected. Check how many clients you have selected. Every client consumes one preview per run, so a 100-client selection burns 100 previews each time you click Run Preview.

Privacy and data handling

The extension is deliberately narrow in what it touches:

• The only data it extracts is the HTML of the email you explicitly choose to send when you click Run Preview.
• That HTML goes directly from your browser to Mailgun's API — the US or EU region you selected — using your own API key. It is not sent to any third-party analytics, ad, or tracking service.
• By default, your API key is held in session storage only and is cleared when the browser closes. If you choose the remember option, the key is instead stored encrypted in your browser profile for up to 30 days.
• The extension does not run in the background, does not read pages until you ask it to, does not collect browsing history, does not collect form inputs from unrelated pages, and does not transmit data to anyone other than Mailgun.

You are responsible for the content you submit to Mailgun Inspect and for having the rights and permissions to extract and submit that content. Don't use the extension to extract or submit content you don't have the right to use.

For the complete picture, see the Mailgun Privacy Policy and the privacy disclosures on the Chrome Web Store listing.