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Deliverability is the measure of how successfully your emails reach recipients’ inboxes rather than spam folders or getting rejected entirely. Even well-crafted emails fail if they never arrive. This guide covers the foundational practices that keep your emails landing in the inbox.

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Authentication Is Non-Negotiable

Email authentication tells receiving mail servers that you are who you claim to be. Without proper authentication, your emails are far more likely to be filtered or rejected.

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Complete Your Domain Setup

Every sending domain in Lettr needs three DNS records configured:
RecordPurposeWhat It Does
CNAMEDomain verificationProves you own the domain and authorizes Lettr to send on your behalf
DKIM (TXT)Email signingAdds a cryptographic signature to every email, proving it wasn’t altered in transit
DMARC (TXT)Policy enforcementTells receivers what to do when authentication fails, and where to send reports
Lettr provides the exact DNS record values when you add a domain. Copy them precisely — even small differences cause verification failures. See Sending Domains for the full setup guide.

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Set Up a Tracking Domain

Using a custom tracking domain (e.g., track.yourdomain.com) instead of a shared tracking domain improves deliverability. Links in your emails will use your own domain, which builds reputation under your brand rather than sharing it with other senders. See Tracking Domains for setup instructions.

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Warm Up New Domains

Mailbox providers treat new sending domains with suspicion. If you send thousands of emails from a domain with no history, spam filters will likely intervene. Warming up means gradually increasing volume so providers can observe consistent, legitimate behavior.
WeekDaily VolumeNotes
150–100Send to your most engaged recipients first
2200–500Monitor bounce and complaint rates closely
3500–2,000Continue only if metrics remain healthy
42,000–5,000Gradually approach your target volume
5+Scale as neededMaintain consistent sending patterns
Sending high volumes immediately from a new domain will damage your reputation before you’ve built one. This damage can take weeks to recover from.

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Warm-Up Tips

  • Start with engaged recipients: Send first to users who regularly open and click your emails. Positive engagement signals help establish reputation.
  • Be consistent: Send every day during warm-up. Sporadic large sends look suspicious.
  • Watch your metrics: If bounce rates exceed 5% or spam complaints exceed 0.3%, slow down and investigate before continuing.
  • Separate transactional and marketing: If you send both types, consider using separate subdomains so marketing reputation issues don’t affect transactional delivery.

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Maintain Clean Recipient Lists

Sending to invalid or unengaged addresses is one of the fastest ways to damage deliverability. Mailbox providers monitor your bounce and complaint rates and will throttle or block senders with poor list hygiene.

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Process Bounces Immediately

Hard bounces mean the address is permanently undeliverable. Continuing to send to hard-bounced addresses signals to mailbox providers that you don’t maintain your lists. Set up a webhook to handle bounce events in real time: 
app.post('/webhooks/lettr', (req, res) => {
  const event = req.body;

  if (event.type === 'email.bounced') {
    const { to, bounceType } = event.data;

    if (bounceType === 'hard') {
      // Remove from your mailing list immediately
      removeFromList(to);
    }
  }

  res.sendStatus(200);
});
See Bounces for detailed bounce handling guidance.

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Use Double Opt-In

Confirm subscriptions before adding addresses to your list. This ensures addresses are valid and that the owner actually wants your emails:
  1. User submits their email address
  2. You send a confirmation email with a unique link
  3. User clicks the link to confirm
  4. Only then add them to your sending list

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Remove Unengaged Subscribers

Recipients who never open your emails drag down your engagement metrics. Mailbox providers use engagement signals (opens, clicks) to decide inbox placement. Periodically remove or re-engage subscribers who haven’t interacted in 6–12 months.

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Content That Avoids Spam Filters

Even with perfect authentication and a clean list, your email content affects deliverability.

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Subject Lines

DoDon’t
Be clear and relevantUse ALL CAPS
Keep under 50 charactersUse excessive punctuation!!!
Personalize when possibleUse spam trigger words (FREE, ACT NOW, URGENT)
Set accurate expectationsMislead about content

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Email Body

DoDon’t
Balance text and imagesSend image-only emails
Include a plain text versionSend HTML only
Use your own domain for linksUse URL shorteners (they’re associated with spam)
Add a visible unsubscribe linkHide or omit the unsubscribe option
Include a physical mailing addressOmit required compliance elements
Every marketing and bulk email should include:
  • Unsubscribe link — One-click unsubscribe is now required by major providers
  • Physical mailing address — Required by CAN-SPAM
  • Reply-to address — Allows recipients to respond
See CAN-SPAM Requirements for detailed legal requirements.

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Monitor Your Metrics

Track these key indicators to catch problems early:
MetricHealthy TargetWarning SignAction
Bounce rate< 2%> 5%Clean your list, validate addresses
Spam complaint rate< 0.1%> 0.3%Review content, check unsubscribe visibility
Open rate> 15%< 10%Improve subject lines, check inbox placement
Click rate> 2%< 1%Improve CTAs and content relevance
Open rates are increasingly unreliable due to Apple Mail Privacy Protection and other privacy features that pre-fetch images. Use click rates and direct engagement as more reliable indicators. See Open & Click Tracking Accuracy for details.

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Testing Before Sending

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Pre-Send Checklist

  1. Send test emails — Send to yourself and colleagues across different email clients (Gmail, Outlook, Apple Mail)
  2. Check rendering — Verify your email looks correct on both desktop and mobile
  3. Verify links — Click every link in the test email to confirm they work
  4. Review content — Read through for spam trigger words and formatting issues
  5. Validate authentication — Check email headers to confirm DKIM is passing
See Test Emails for how to send test emails through Lettr.

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Common Deliverability Issues

  • Verify all authentication records (DKIM, DMARC) are passing
  • Review your email content for spam triggers
  • Check that you have a visible unsubscribe link
  • Ensure your domain has built sufficient reputation (warm up if new)
  • See Emails Landing in Spam for step-by-step diagnosis
  • Implement double opt-in for new subscribers
  • Validate email addresses at the point of collection
  • Remove subscribers who haven’t engaged in 6+ months
  • Process bounce webhooks immediately — don’t batch them
  • See Bounce Diagnosis for interpreting bounce codes
  • Improve subject lines — test different approaches
  • Send at times when your audience is most active
  • Segment your audience for more relevant content
  • Remove unengaged subscribers to improve overall metrics
  • Check if Apple MPP is inflating or deflating your numbers
  • Check if you’re sending too fast during domain warm-up
  • Verify your domain reputation hasn’t been damaged
  • Look for email.deferred webhook events for specific reasons
  • Some delays are normal — receiving servers may throttle new senders

Sending Reputation

Build and protect your sender reputation

List Hygiene

Maintain healthy recipient lists

Domain Setup

Configure and verify your sending domains

Bounces

Understand bounce types and handling