How to Monitor Email Sending and Deliverability (SMTP, APIs, and Webhooks)

Email is the most important channel almost nobody monitors. Your app sends password resets, receipts, magic links, invoices, and onboarding sequences — and every one of them is fire-and-forget. You call an API, it returns 200, you move on. But "the API accepted the request" is not the same as "the message reached the inbox." Somewhere between your code and your customer's mailbox, the message can be queued forever, bounced, marked as spam, or dropped because a DNS record expired. And here is the cruel part: nobody files a ticket when an email doesn't arrive.

This is the silent-failure problem. A user who never receives a confirmation email assumes it is slow, checks their spam folder, gives up, and churns. A finance team that stops receiving invoices notices weeks later. You find out about broken email through anecdotes and lost revenue, not through alerts. The fix is to stop trusting that "no news is good news" and to actively monitor every stage of the pipeline. This guide shows you how — using HTTP and heartbeat checks, which is what tools like CronAlert actually do.

The email pipeline has five places to break

"Monitor email" sounds like one task, but the path from your code to an inbox crosses several independent systems, each with its own failure mode. Treat them separately:

Your email provider (SES, SendGrid, Postmark, Resend, Mailgun) — a third-party dependency that can have outages.
Your app's send path — the code, credentials, and configuration that actually call the provider.
Scheduled and queued jobs — digests, drip campaigns, retries, and background senders that run on a timer.
Bounce and complaint webhooks — the endpoint your provider POSTs delivery events to.
DNS and authentication records — SPF, DKIM, DMARC, and MX, plus SSL on any mail-related web endpoints.

A single missed alert in any one of these can take down email while every dashboard stays green. Let's walk through how to put a check on each.

1. Monitor your email provider as a third-party dependency

Your transactional email provider is infrastructure you don't control, which means its outages silently become yours. When SES throttles a region or SendGrid has an API incident, your sends fail through no fault of your code. You want to know immediately whether a delivery problem is the provider's fault or yours, because the response is completely different — wait it out versus fix a bug versus fail over to a backup.

Most providers publish a status or health endpoint. Point an API endpoint monitor at it and treat it exactly like any other third-party dependency. Use a content or keyword check so the monitor passes only when the JSON body actually reports "operational," not merely when the page returns 200 — a status page can serve a perfectly healthy 200 while announcing a major outage in its body. CronAlert's keyword and regex matching (plus SHA-256 content hashing for change detection) make this a one-field configuration.

If you use more than one provider — say a primary and a fallback — monitor both. The whole point of a fallback is that it works on the day your primary doesn't, and an unmonitored fallback has a habit of being broken precisely when you finally reach for it.

2. Monitor your own send path with a health endpoint

The provider being up tells you nothing about your ability to use it. The most common email outages are self-inflicted: a rotated API key that never made it into the production environment, an expired credential, a misconfigured "from" domain, a region mismatch, a library upgrade that changed a default. None of these show up on the provider's status page, and none of them throw an error until you actually try to send — which, for a rarely-used flow like password reset, might be hours after the deploy that broke it.

The fix is a dedicated HTTP health endpoint in your application that performs a lightweight send-readiness check and nothing more. It should not send a real email on every call — that would be expensive and noisy — but it should verify the things that actually break:

Required credentials are present and non-empty in the environment.
The configured "from" address and sending domain are set.
The provider's API is reachable (a cheap authenticated call, like fetching account info or sender identities).
Any local queue or SMTP relay your app talks to accepts a connection.

Return 200 when everything is ready and 500 (with a short diagnostic body) when anything is missing. Then have CronAlert hit /health/email on an interval. The moment a deploy ships without the right secret, the check fails and you hear about it in minutes — not when the first customer can't log in. Keep the endpoint behind a secret path or header so it isn't publicly enumerable.

Covering raw SMTP without a TCP-port monitor

If you run your own SMTP relay or depend on a specific SMTP server, you might want a check that the port itself is answering. Be aware of the constraint: CronAlert does not probe raw TCP ports like 25, 465, or 587 — it speaks HTTP/HTTPS and heartbeats only. A dedicated TCP-port monitor is a different kind of tool.

You can still get effective coverage with the health-endpoint pattern. Add a route to your app — for example /health/smtp — that opens a connection to your SMTP host, completes the handshake (an EHLO and optionally STARTTLS), and immediately disconnects without sending anything. If the handshake succeeds, return 200; if the connection times out or is refused, return 503 with the error. Now an ordinary HTTP monitor verifies your SMTP port indirectly, and you get the alerting, history, and multi-channel notifications of HTTP monitoring for a service that doesn't speak HTTP. Mind your timeout thresholds here: SMTP handshakes can be slow, so give the endpoint a generous internal timeout and a monitor timeout that won't false-alarm on a normal-but-sluggish connection.

3. Monitor scheduled and queued email jobs with heartbeats

Plenty of email never comes from a user clicking a button. Daily digests, weekly summaries, dunning emails, drip sequences, and retry workers all run on schedules or off a queue. These are the most dangerous failures of all, because a job that silently stops running produces no error to catch — there is simply an absence of activity, and absence is invisible to ordinary monitoring.

This is exactly what heartbeat (cron) monitoring is for. Instead of CronAlert reaching out to your job, your job reaches out to CronAlert: at the end of a successful run, it sends an HTTP request to a unique heartbeat URL. CronAlert expects that ping within a window you define, and if the ping doesn't arrive — because the worker crashed, the scheduler broke, or the job exited early before sending — it alerts you. You are monitoring the non-event, which is the only thing that matters for a background task.

A few practical tips for email jobs specifically:

Ping after the send, not before. Put the heartbeat call at the very end of the job, after the batch has actually been dispatched, so a crash midway through doesn't report success.
Use the start/finish signal for long batches. For a digest that takes 20 minutes, signal "started" and "finished" so a job that hangs is caught, not just one that never begins. This same approach applies to any batch job monitoring.
Set the grace window to match real variance. A nightly job that sometimes runs long needs enough slack to avoid false alarms but not so much that a true failure goes hours without notice.

The same pattern protects retry queues and any background worker that drains pending emails — if the worker dies, the heartbeat stops, and you know before the backlog becomes a customer-facing delay.

4. Monitor your bounce and complaint webhook receiver

Good email hygiene depends on processing the events your provider sends back: bounces, complaints (spam reports), deliveries, and unsubscribes. Your provider POSTs these to a webhook endpoint you host, and you use them to suppress bad addresses and protect your sender reputation. But that endpoint is itself a service that can go down — and when it does, you stop suppressing bounces, keep mailing dead addresses, and quietly torch your deliverability.

Monitor the receiver like any other critical endpoint. Webhook receiver monitoring means putting a health check on the route your provider POSTs to (or a sibling GET route that exercises the same handler path) so you know it is reachable and returning success. If the endpoint starts 500-ing or times out, your provider will retry for a while and then start dropping events — so catching the outage in minutes, not days, is the difference between a clean suppression list and a poisoned one.

5. Monitor DNS, authentication records, and SSL

The final stage is the one most teams forget until a major mailbox provider starts rejecting their mail: the DNS records that authenticate your domain. SPF, DKIM, DMARC, and MX are what tell receiving servers your mail is legitimate. They break in undramatic ways — someone edits a TXT record and fat-fingers it, a key rotation removes the old DKIM selector too early, a registrar migration drops records, an SPF record silently exceeds its lookup limit. Suddenly your perfectly-sent mail lands in spam or is rejected outright, and nothing in your app or your provider reports an error.

Set up DNS monitoring on the records that gate deliverability so you are alerted when an SPF, DKIM, DMARC, or MX value changes or disappears. Pair it with SSL certificate monitoring on any mail-related web endpoints — your unsubscribe pages, tracking domains, hosted webhook receiver, and the health endpoints above. An expired certificate on a tracking or click domain quietly breaks links and dings engagement metrics, which mailbox providers read as a reputation signal. These two checks are cheap insurance against an entire class of silent outages that have nothing to do with your code.

Surfacing deliverability signals through a health endpoint

Beyond "is mail going out," you want to know "is mail being accepted." Bounce rate, complaint rate, and blocklist status are the leading indicators of deliverability trouble, and they degrade gradually — by the time you notice fewer signups, a mailbox provider may already be throttling you. These numbers live in your provider's API and your own records, not in a simple ping, so make them readable by a monitor the same way you did for the send path: build an endpoint that queries the metrics and returns a non-200 status when any of them crosses a threshold you set (say, complaint rate above 0.1% or any active blocklist entry). CronAlert reads the pass/fail and alerts you while you still have time to react — pause a campaign, clean your list, or open a remediation request — rather than after the damage is done.

A practical setup checklist

Putting it all together, here is a sequence you can implement in an afternoon:

Provider status: add an HTTP monitor on your email provider's status/health endpoint with a keyword check for "operational." Repeat for any fallback provider.
Send-path health: build /health/email that verifies credentials, sender config, and provider reachability; monitor it on a 1–3 minute interval.
SMTP handshake (if applicable): add /health/smtp that opens and closes an SMTP connection, and monitor it as an HTTP check with a generous timeout.
Scheduled jobs: add a heartbeat ping to the end of every digest, drip, dunning, and retry job; set grace windows to match real run times.
Webhook receiver: health-check the endpoint your provider POSTs bounce and complaint events to.
DNS and SSL: monitor SPF, DKIM, DMARC, and MX records, plus SSL on every mail-related web endpoint.
Deliverability: expose a metrics endpoint that fails when bounce rate, complaint rate, or blocklist status crosses your thresholds.
Routing: send these alerts to a channel a human actually watches — email is too important to bury in a noisy firehose.

With CronAlert's free plan you get 25 monitors at a 3-minute interval, SSL and content checks, the full REST API, and a 7-day history — enough to cover the entire checklist above for a typical app. Upgrading to Pro drops you to a 1-minute interval and unlocks every alert channel (email, Slack, Discord, Teams, Telegram, PagerDuty, Opsgenie, Splunk On-Call, webhooks, and PWA push), so a broken send path can page whoever is on call. Every plan includes the full API and MCP support for Claude Code, Cursor, and Windsurf, so you can manage all of these monitors as code.

Frequently asked questions

Can CronAlert connect directly to my SMTP server on port 25, 465, or 587?

No. CronAlert performs HTTP/HTTPS checks and heartbeat (cron) checks only — it does not open raw TCP connections to SMTP ports. The recommended pattern is to expose a small HTTP health endpoint in your application that attempts an SMTP handshake or test send and returns 200 on success and 500 on failure. CronAlert hits that endpoint and alerts you when it fails, giving you SMTP coverage indirectly without a TCP-port monitor.

Why do email failures so often go unnoticed?

Email is a one-way, fire-and-forget channel. When a password-reset or receipt email fails to send, nobody files a ticket — the user simply assumes the email is slow or checks their spam folder. Your application logs a success because the API call returned, even if the message later bounced or was silently dropped. By the time support hears about it, you may have lost days of signups or payments. Active monitoring of the send path and provider is the only reliable way to catch these silent failures.

How do I monitor scheduled or queued email jobs like daily digests?

Use a heartbeat (cron) monitor. After your digest, drip, or retry job finishes successfully, have it send an HTTP request to a unique heartbeat URL. CronAlert expects that ping on a schedule and alerts you when it does not arrive — which is exactly the failure mode you care about, because a job that silently stops running produces no error of its own. This catches crashed workers, broken schedulers, and jobs that exit early before sending.

Should I monitor my email provider's status page?

Yes. Your transactional email provider — SES, SendGrid, Postmark, Resend, Mailgun, and others — is a third-party dependency, and its outages become your outages. Most providers expose a public status or health endpoint you can monitor as an HTTP check. Watching it lets you distinguish "the provider is down" from "our integration is broken," which dramatically speeds up incident triage and tells you whether to wait or to fail over to a backup provider.

How can I monitor deliverability signals like bounce and complaint rates?

Deliverability metrics live in your provider's API and your own bounce/complaint records, not in a simple HTTP ping. The practical approach is to build a health endpoint that queries those numbers and returns a non-200 status when bounce rate, complaint rate, or blocklist status crosses a threshold you define. CronAlert then reads that endpoint as a pass/fail check and alerts you when deliverability degrades, before a mailbox provider starts throttling or blocking your domain.

Start monitoring your email pipeline today

Broken email is uniquely expensive because it fails in silence — no errors, no tickets, just lost users and revenue you only notice in hindsight. You don't need a special "email monitor" to fix this; you need HTTP and heartbeat checks pointed at the right places: your provider's status, your own send-path health endpoint, your scheduled jobs, your webhook receiver, and your DNS and SSL. CronAlert does exactly those checks, on the edge, with no agent to install. Create a free CronAlert account and put a check on every stage of your email pipeline before the next silent failure costs you a customer.