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Lettr monitors your API usage and the DNS health of your verified domains, and alerts you when something breaks. Alerts help you catch integration issues, misconfigured domains, or unexpected error patterns before they impact your users.

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API Failure Rate Alerts

Lettr runs two automated checks against your API traffic:

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15-Minute Rolling Window

Detects sudden spikes in error rates. If the failure rate in the last 15 minutes exceeds the threshold, an alert is triggered immediately. This catches acute issues like a broken integration deployment or an expired API key being used in production.

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Daily Low-Volume Check

Detects sustained elevated error rates across lower-traffic periods. Even if individual 15-minute windows don’t breach the threshold, a consistently elevated daily failure rate triggers an alert. This catches chronic issues like a misconfigured domain or a gradual increase in invalid recipients.
Alert cooldowns are asymmetric — a 15-minute alert suppresses the daily alert for the same issue, but not vice versa. This prevents duplicate notifications while ensuring urgent issues are always surfaced.

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Domain DNS Health Alerts

Once a domain is verified, Lettr keeps watching it. A daily health check runs for every verified domain on your team — sending, tracking, inbound, and storage — and alerts you if a DNS record that used to work stops resolving. This catches the most common silent failure: a record that was deleted or repointed at your DNS provider long after setup. What each check looks at: When a check fails:
  • In-app — a notification appears in the notification center, a red Not working badge shows next to the domain in list views, a warning panel appears on the domain’s detail page explaining the consequence (for example, broken open/click tracking or images no longer loading), and a critical alert shows on the dashboard.
  • Email — if the email channel is enabled, the team owner receives one digest email per check run listing every domain that broke.
  • Webhook — if a webhook URL is configured, a domain_dns_regression event is delivered with the affected domain, its type, and the failing records. Webhook alerts are HMAC-signed when a signing secret is configured.
A cooldown (configurable from 1 to 720 hours, 24 hours by default) suppresses repeat alerts for the same failing record. The cooldown is per record per domain, so a newly broken record always alerts even while another is still failing.
DNS health monitoring is informational — it never disables a domain or stops your sending. Once you fix the DNS record, the warning clears automatically on the next check; you don’t need to re-verify the domain manually.
Enable or disable DNS health alerts and adjust the cooldown under Settings → Alerts → Domain DNS Health.

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Alert Channels

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Email

Email alerts are enabled by default and sent to the team owner’s email address. Alert emails are sent through Lettr’s own transactional email system.

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Webhook

Configure an HTTPS webhook URL to receive alerts as JSON POST requests. This is useful for integrating with incident management tools like PagerDuty, OpsGenie, or Slack.
Webhook URLs must be publicly accessible HTTPS endpoints. Private IP addresses and internal network URLs are blocked for security (SSRF protection).

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Testing Your Webhook

After configuring a webhook URL, use the Test Webhook button to send a test payload and verify connectivity. The test confirms your endpoint is reachable and returns a success response.

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Configuring Alerts

1

Navigate to Alert Settings

Go to Settings → Alerts.
2

Enable or Disable Alerts

Toggle alerts on or off for each alert type. You can enable the 15-minute check independently from the daily check.
3

Choose Alert Channels

Select which channels should receive alerts — email, webhook, or both.
4

Configure Webhook (Optional)

If using the webhook channel, enter your HTTPS endpoint URL and click Test Webhook to verify connectivity.
5

Save

Click Save to apply your alert configuration.

Try it in the app

Follow an interactive walkthrough of this guide inside Lettr.

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Best Practices

Even if your integration is stable, alerts provide early warning when something changes — a DNS misconfiguration, an expired certificate, or a code deployment that introduces errors.
Route webhook alerts to your incident management system (PagerDuty, OpsGenie, Slack) so the right person is notified immediately.
When you receive an alert, check the Analytics dashboard for error breakdowns to identify the root cause — whether it’s a specific domain, API key, or error type.

Analytics

Monitor sending metrics and error breakdowns

Errors & Retries

Understand API error codes and retry patterns