Setting Up Deliverability Alerts: What to Monitor

Why Real-Time Alerts Matter

Email deliverability problems compound quickly:

• A blacklist hit spreads to secondary lists within hours
• High complaints for one campaign affect future campaigns
• Authentication failures cause immediate rejection
• Reputation damage takes weeks to repair

Early detection—within hours, not days—dramatically reduces recovery time.

Practitioner note: The worst deliverability crises I've handled all started the same way: something went wrong on Friday afternoon and nobody noticed until Monday. By then, three days of damage required six weeks to fix.

Essential Alerts to Configure

Alert 1: Spam Complaint Rate

Threshold: Above 0.1% (warning), Above 0.3% (critical)

Why: Gmail's threshold is 0.3%. Exceeding it triggers spam filtering. Catch it at 0.1% to fix before critical.

Tools:

• ESP dashboards (most send alerts)
• Google Postmaster Tools (check manually or scrape)
• Custom webhook from ESP to Slack/PagerDuty

Response: Identify the campaign, pause sending, analyze the complaint source.

Alert 2: Bounce Rate

Threshold: Above 2% (warning), Above 5% (critical)

Why: High bounces indicate list quality issues or technical problems. Hard bounces damage IP/domain reputation directly.

Tools:

• ESP built-in alerts
• Webhook notifications per campaign
• Custom monitoring dashboards

Response: Check for bad list import, validation issues, or DNS problems.

Alert 3: Blacklist Appearances

Threshold: Any listing on major lists (critical)

Why: Major blacklists (Spamhaus, Barracuda) cause immediate delivery problems across multiple ISPs.

Tools:

• MXToolbox Blacklist Monitoring ($99+/month)
• GlockApps DMARC Analyzer
• HetrixTools (free tier available)
• Custom scripts checking blacklist APIs

Response: Initiate delisting request, investigate root cause, pause sending if severe.

Alert 4: Authentication Failures

Threshold: SPF/DKIM pass rate below 99%

Why: Authentication failures mean mail is failing alignment, potentially landing in spam or being rejected.

Tools:

• DMARC aggregate report parsing (via dmarcian, Postmark, GlockApps)
• Google Postmaster authentication dashboard
• Custom DMARC report processor

Response: Check DNS records, verify ESP configuration, investigate unauthorized senders.

Alert 5: Reputation Drops

Threshold: Any drop from High to Medium (warning), Drop to Low/Bad (critical)

Why: Reputation changes predict future deliverability. Catching Medium before Low gives you time to act.

Tools:

• Manual Postmaster Tools check (no native alerts)
• Third-party scrapers (Postmaster API is limited)
• Validity Everest (enterprise)

Response: Review recent campaigns, check complaint/bounce trends, reduce volume to engaged subscribers.

Alert Configuration by Tool

ESP Built-In Alerts

Most ESPs offer basic alerting:

SendGrid:

• Settings → Alerts → Configure thresholds
• Supports bounce, block, spam report alerts
• Webhook integration available

Mailgun:

• Account Settings → Webhooks
• Events: complaints, bounces, unsubscribes
• Forward to Slack, PagerDuty, or custom endpoint

Postmark:

• Alerts → New Alert
• Bounce, spam complaint thresholds
• Email or webhook delivery

Klaviyo:

• Limited built-in alerts
• Use API + custom monitoring for advanced alerts

MXToolbox Monitoring

Setup:

1. Create account at mxtoolbox.com
2. Add your sending domains and IPs
3. Configure blacklist monitoring
4. Set email/webhook notifications

Coverage:

• 100+ blacklists
• DNS record monitoring
• MX configuration checks

Cost: Free for basic, $99/month+ for comprehensive monitoring

GlockApps Alerts

Setup:

1. Enable Auto-tests in GlockApps
2. Set threshold for inbox placement (e.g., below 85%)
3. Configure email notifications

Coverage:

• Inbox placement drops
• DMARC report anomalies
• Authentication failures

Custom Webhook Alerts

For advanced monitoring, set up webhooks to aggregate ESP data:

// Example: Node.js webhook receiver
app.post('/email-webhook', (req, res) => {
  const event = req.body;

  if (event.type === 'complaint') {
    // Calculate complaint rate
    const rate = complaints / delivered;
    if (rate > 0.001) { // 0.1%
      sendSlackAlert(`Complaint rate warning: ${rate * 100}%`);
    }
  }

  if (event.type === 'bounce' && event.bounce_type === 'hard') {
    hardBounceCount++;
    if (hardBounceRate > 0.02) { // 2%
      sendPagerDutyAlert('Critical: Hard bounce rate exceeded 2%');
    }
  }

  res.sendStatus(200);
});

Integration with Monitoring Platforms

Datadog:

• Receive ESP webhooks
• Create dashboards for email metrics
• Set anomaly detection alerts

PagerDuty:

• Escalation policies for critical alerts
• On-call rotation for overnight issues
• Incident management workflows

Slack:

• Channel for deliverability alerts
• Integration with ESP webhooks
• Searchable alert history

Alert Threshold Reference

Metric	Warning	Critical	Check Frequency
Spam complaint rate	>0.1%	>0.3%	Per campaign
Hard bounce rate	>2%	>5%	Per campaign
Soft bounce rate	>5%	>10%	Per campaign
Blacklist appearance	Any minor list	Any major list	Hourly
Authentication pass rate	<99%	<95%	Daily
Google reputation	Drop to Medium	Drop to Low/Bad	Daily
Inbox placement	<85%	<70%	Pre-campaign

Response Procedures

For Complaint Spike

1. Identify the triggering campaign (within 1 hour)
2. Pause similar campaigns (within 2 hours)
3. Analyze recipient segment (within 4 hours)
4. Implement fix: list cleaning, content change, etc.
5. Resume sending to engaged segment only

For Blacklist Appearance

1. Confirm listing and identify list (immediately)
2. Submit delisting request (within 1 hour)
3. Investigate root cause (within 4 hours)
4. If Spamhaus or major list: pause sending
5. Monitor delisting progress daily

For Authentication Failure

1. Check DNS records for errors (immediately)
2. Verify ESP configuration (within 2 hours)
3. Check for unauthorized senders in DMARC reports
4. Fix DNS or configuration issues
5. Monitor for resolution in next DMARC report cycle

Building an Alert Runbook

Document procedures for each alert type:

## Complaint Rate Alert Runbook

### Trigger
Complaint rate exceeds 0.1% for any campaign

### Immediate Actions
1. Open ESP dashboard, identify campaign
2. Check Postmaster Tools for Gmail reputation impact
3. Note campaign details: segment, content, send time

### Investigation
1. Compare to baseline complaint rate
2. Check recipient segment for issues
3. Review content for spam triggers
4. Check if unsubscribe worked properly

### Resolution
1. Add complainers to suppression
2. Clean segment if list issue
3. Adjust content if content issue
4. Document incident

### Follow-up
1. Monitor next 3 campaigns closely
2. Check Postmaster Tools in 48 hours
3. Review segment performance weekly

If you need help building a comprehensive alerting system or want expert guidance on response procedures, schedule a consultation to set up monitoring that catches problems before they become crises.

Sources

• MXToolbox: Monitoring Services
• SendGrid: Webhooks
• Mailgun: Webhooks
• GlockApps: Auto Tests

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v1.0 · March 2026