tutorial

Monitoring Resend with Vigilmon: Email API Uptime, Webhook Delivery, Sending Domain Health & SSL Certificate Checks

How to monitor Resend email delivery service with Vigilmon — API availability, webhook endpoint health, sending domain DNS monitoring, SSL certificate checks, and delivery status alerting for 2026.

Resend is a developer-focused transactional email API built for modern applications — providing a clean REST API, React email templates, webhook delivery events, and excellent deliverability infrastructure. When Resend has an incident or your integration degrades, transactional emails do not go out: password resets fail, welcome emails are never delivered, notification emails queue indefinitely. Vigilmon gives you an external monitoring layer over your Resend integration: API availability, your webhook receiver health, sending domain DNS configuration, and SSL certificates on your application domain.

What You'll Build

  • A monitor on the Resend API for external availability tracking
  • A webhook receiver health check ensuring your endpoint accepts Resend events
  • A domain DNS check confirming your sending domain records are resolving correctly
  • SSL certificate monitoring for your application domain
  • Alerting tuned for email delivery infrastructure

Prerequisites

  • A Resend account with at least one verified sending domain
  • A webhook receiver endpoint (optional but recommended for delivery tracking)
  • A free account at vigilmon.online

Step 1: Monitor the Resend API

Resend's REST API at api.resend.com is the single point through which all your application's transactional email flows. An API outage means no emails go out — not just delayed, but completely blocked. Verify the API is reachable:

curl -I https://api.resend.com/emails \
  -H "Authorization: Bearer YOUR_API_KEY"

A healthy API returns 200 for authenticated requests. For an unauthenticated availability probe:

  1. Log in to Vigilmon → Add Monitor → HTTP.
  2. URL: https://api.resend.com/domains (returns 401 without auth).
  3. Check interval: 60 seconds.
  4. Expected status: 401 (confirms API host is reachable and authenticating).
  5. Label: Resend API - Availability.
  6. Click Save.

Why probe with 401: The 401 response from an unauthenticated request proves the API is reachable, TLS handshakes succeed, and the application layer is processing requests. A timeout or 502 means Resend's API infrastructure is degraded.

Step 2: Monitor Your Webhook Receiver

Resend fires webhook events for email lifecycle milestones: email.sent, email.delivered, email.bounced, email.complained, email.clicked. If your webhook receiver is down, you lose visibility into delivery status — bounces accumulate unnoticed, unsubscribe events are not processed, and your sending reputation degrades without any local signal.

Add a monitor on your webhook endpoint:

  1. Add Monitor → HTTP.
  2. URL: https://your-app.example.com/webhooks/resend.
  3. Check interval: 60 seconds.
  4. Expected status: 200 or 405 (GET on a POST-only route returns 405 — confirms the endpoint is alive and routed correctly).
  5. Label: Resend Webhook Receiver.
  6. Click Save.

Resend webhook signatures: Resend signs webhook payloads using svix, sending a svix-signature header. Your receiver should verify this before processing. A simple GET health check does not trigger signature validation but confirms the endpoint is reachable.

Step 3: Monitor Your Application's Email-Dependent Pages

Email-dependent user flows — password reset, email verification, account onboarding — fail silently when Resend is down. Users hit "Send reset link" and nothing arrives. Add monitors on pages that trigger transactional emails so you can correlate UI availability with email delivery:

curl -I https://your-app.example.com/forgot-password
curl -I https://your-app.example.com/register

Add monitors for your critical email trigger pages:

  1. Add Monitor → HTTP.
  2. URL: https://your-app.example.com/forgot-password.
  3. Check interval: 60 seconds.
  4. Expected status: 200.
  5. Keyword: password or email (a term present in the page content).
  6. Label: App - Password Reset Page.
  7. Click Save.

Repeat for /register or any other high-value email trigger endpoint. Monitoring the page separately from the API confirms which layer is failing when an email flow breaks.

Step 4: Monitor SSL Certificates

Resend requires your sending domain to have valid DNS records (SPF, DKIM, DMARC). Your application domain also needs a valid TLS certificate — an expired cert on your app domain prevents users from reaching the pages that trigger transactional emails in the first place.

Add SSL monitoring:

  1. Add Monitor → SSL Certificate.
  2. Domain: your-app.example.com.
  3. Alert when expiry is within: 30 days.
  4. Alert again: 14 days, 7 days, 3 days, 1 day.
  5. Click Save.

If you send from a custom domain (mail.your-app.com) and host any resources at that subdomain, add a second SSL monitor. Email infrastructure and application infrastructure need separate certificate coverage.

Step 5: Track Resend's Status Page

Resend publishes system status at resend-status.com. Add a keyword monitor so Vigilmon alerts you immediately when Resend reports degradation — giving you an automatic signal to hold email queues or communicate to users:

  1. Add Monitor → HTTP.
  2. URL: https://resend-status.com/.
  3. Check interval: 5 minutes.
  4. Expected status: 200.
  5. Keyword: All Systems Operational (present during normal operation; absent during incidents).
  6. Label: Resend Status Page.
  7. Click Save.

Alert correlation: When the Resend API monitor fires and the status page keyword also fails, the problem is Resend's infrastructure. When only your application pages fire, the problem is your deployment. When only the webhook receiver fires, it is your app server. Separate monitors give you precise diagnostic information.

Step 6: Configure Alerting

In Vigilmon under Settings → Notifications, configure alert channels:

| Monitor | Trigger | Action | |---|---|---| | Resend API - Availability | Non-2xx/4xx or timeout | Resend API incident; queue emails locally if possible; check status page | | Resend Webhook Receiver | Non-200/405 or timeout | App server down; delivery events lost; bounce handling broken; fix app server | | App - Password Reset Page | Non-200 or keyword missing | Email trigger page broken; users cannot reset passwords; investigate deployment | | Resend Status Page | Keyword missing | Platform-side incident; inform support team; hold marketing sends | | SSL Certificate | < 30 days to expiry | Renew certificate; expired cert blocks user access to email trigger pages |

Alert sensitivity for email infrastructure: Set the API and webhook monitors to alert after 1 consecutive failure. Email failures are often invisible to users (the UI says "check your inbox" regardless) — fast detection prevents a trickle of failed sends from becoming a flood of support tickets.

Common Resend Failure Modes and What Vigilmon Catches

| Scenario | Vigilmon monitor | |---|---| | Resend API incident (upstream) | API Availability fires; Status Page keyword fails | | Webhook receiver app crash | Webhook Receiver fires; delivery events lost silently | | Bad deploy breaks password reset page | Password Reset Page monitor fires | | SSL certificate expires on app domain | SSL monitor alerts at 30-day threshold | | DNS misconfiguration on sending domain | Delivery rates drop (not directly caught by HTTP monitors) | | App server OOM crash | Webhook Receiver and Password Reset Page fire simultaneously | | Database connection pool exhausted | API calls succeed but email send endpoint returns 500 | | Resend rate limit reached | API returns 429; API monitor fires if not distinguished from expected 4xx | | Network partition isolating your app | All monitors fire simultaneously from Vigilmon's external perspective | | Resend maintenance window | Status Page keyword fails; API may be intermittent |

Why Monitor Resend with an External Tool

Transactional email failures are uniquely silent. Your application code calls Resend's API and considers the job done — it does not wait for delivery confirmation. When Resend's API is down, your application may not surface an error at all if the send is fire-and-forget. Users experience the failure minutes later when the expected email never arrives, and the support ticket arrives long after the root cause has changed. Vigilmon detects the API outage the moment it starts — before any user notices, before any support ticket is filed, before your team starts debugging why password resets are failing.

Email infrastructure needs the same monitoring rigor as your database or application server. It is load-bearing infrastructure that most teams do not monitor because they cannot see it failing.

Resend makes transactional email simple and reliable. Vigilmon ensures you know immediately when that reliability has a gap — from the API layer through your webhook receiver to the user-facing pages that depend on email delivery.

Start monitoring your Resend integration in under 5 minutes — register free at vigilmon.online.

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