## TL;DR
Your SendGrid event webhook is failing with 401 because your endpoint is rejecting SendGrid's POSTs, usually a missing or mismatched auth check on your side. SendGrid keeps retrying, then gives up. The fix: make sure your endpoint does not demand a login for this path, and validate requests with SendGrid's signed webhook verification instead of a generic bearer check. Once the path accepts SendGrid's posts, events flow again.

## The error
```text
POST /webhooks/sendgrid/events returned 401 Unauthorized
SendGrid activity feed: "Event webhook failed: 401"
```

## Steps
1. Confirm the failure is on your side. Check your app logs for POSTs to the webhook path around the time SendGrid reported them. Expected output: log lines showing the POST arriving and your app returning 401.
2. Find what demands auth. Look at the route's middleware: an auth guard, an API gateway rule, or a platform-level login requirement. Expected output: one identifiable rule (for example, all routes require a session) covering the webhook path.
3. Exempt the webhook path from login-style auth. Webhooks cannot log in; they identify themselves by signature, not by session. Expected output: the route config shows the webhook path excluded from session auth.
4. Verify with SendGrid's signature instead. Use the official Event Webhook verification: check the signature header against your verification key. Expected output: valid test posts pass verification; forged posts are rejected.
5. Send a test event from the SendGrid dashboard. Expected output: your endpoint logs a 200 and the event appears in your system.

## When to use
- SendGrid's activity feed or your logs show webhook posts returning 401
- Engagement events (delivered, opened, clicked, bounced) stopped arriving
- The webhook worked, then broke after an auth or gateway change

## When not to use
- Emails are not being delivered to inboxes: that is a sending and reputation problem, not the webhook
- You get 404s: the path is wrong, not unauthorized
- You get 500s: your handler is crashing, fix the crash

## Tool compatibility
SendGrid Event Webhook on any plan that includes it. The signed-verification approach works with the official SendGrid libraries for Node, Python, Ruby, Go, Java, and PHP, or a hand-rolled check in any stack.

## Variant phrasings
### sendgrid webhook 401
Generic auth rejection of the event webhook path.
### sendgrid event webhook not firing
Often the same underlying cause: the endpoint rejects the POST.
### sendgrid signed webhook verification
The correct way to authenticate webhook posts.
### sendgrid webhook authentication failed
Check gateway rules before blaming SendGrid.

## Why it happens
SendGrid posts events to your URL as a server-to-server call with no user session and no login. If your route requires one (a global auth middleware, an API gateway policy, or a hosting platform's access control), every post gets a 401. A second common cause: someone added a bearer check on the route and SendGrid does not send that credential. SendGrid's own answer is the signed webhook: each request carries a signature you verify with your key, which proves the post is really from SendGrid without any login dance.

## Edge cases
- 401s started after a deploy: a new global middleware caught the route. Pin the exemption in code, not in a dashboard toggle someone can forget.
- Works in staging, 401s in production: the production gateway has an extra auth policy. Compare the two configs line by line.
- You verify the signature and still get 401: the 401 is coming from a layer in front of your code (CDN, WAF, gateway). Check each layer's logs.
- Rotated the verification key: update the stored key everywhere the check runs, or every post fails verification at once.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_RGabBDlaR_MsqKXRTYkVDA