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Gmail processes over half of all consumer email, so Gmail-specific delivery problems can have an outsized impact on your sending metrics. This guide covers Gmail’s unique filtering behaviors, how to use Google Postmaster Tools, and how to resolve common blocks.

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Understanding Gmail’s Tab System

Gmail sorts incoming email into tabs: Primary, Promotions, Social, Updates, and Forums. Tab placement is not the same as spam — emails in the Promotions tab are delivered successfully, but they get less visibility.

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What Determines Tab Placement

Gmail uses machine learning to classify messages. The main signals are:
SignalPrimaryPromotions
Sender relationshipFrequent 1:1 conversationsBulk or automated sends
Content stylePlain text, conversational toneHTML-heavy, images, CTAs
Unsubscribe headerAbsent (transactional)Present (marketing)
Engagement historyRecipient regularly opens and repliesRecipient rarely interacts
Links and imagesFew or noneMultiple links, large images

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Improving Primary Tab Placement

For transactional emails that are landing in Promotions:
  • Simplify the HTML — use minimal styling, fewer images, and avoid marketing-style layouts for receipts, alerts, and notifications.
  • Remove promotional elements — banners, upsells, social media links, and footer marketing content push messages toward Promotions.
  • Use a plain From name — a personal name (e.g., “Jane from Acme”) performs better than a brand name for Primary placement.
  • Send from a separate subdomain — isolate transactional email on a subdomain like notify.yourdomain.com so its reputation isn’t affected by marketing sends.
Tab placement is per-user. If a recipient manually drags your email from Promotions to Primary, Gmail learns to place future messages from you in Primary for that user.
Do not try to trick Gmail’s tab classifier by stripping unsubscribe headers from marketing email. This violates Google & Yahoo Requirements and will damage your reputation.

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Setting Up Google Postmaster Tools

Google Postmaster Tools (GPT) is the most important diagnostic tool for Gmail delivery. It shows domain reputation, spam rate, authentication status, and delivery errors — data that is not available anywhere else.
1

Go to Google Postmaster Tools

Visit postmaster.google.com and sign in with a Google account.
2

Add your sending domain

Click the + button and enter your sending domain (e.g., mail.yourdomain.com). Use the exact domain you send from in Lettr, including the subdomain if applicable.
3

Verify domain ownership

Google provides a TXT record to add to your DNS. Add it at your DNS provider:
google-site-verification=xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
Click Verify once the record is published. Propagation usually takes a few minutes but can take up to 48 hours.
4

Wait for data

Data appears after you send a sufficient volume to Gmail addresses (typically 100+ messages per day). Dashboards populate within 24–48 hours of reaching this threshold.

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Key Dashboards

DashboardWhat It ShowsWhat to Watch For
Spam RatePercentage of emails marked as spam by recipientsKeep below 0.1%. Action required above 0.3%.
Domain ReputationOverall trust level: High, Medium, Low, BadLow or Bad causes most email to go to spam
IP ReputationTrust level of sending IP addressesRelevant if using dedicated IPs
AuthenticationDKIM, SPF, and DMARC pass ratesShould be 100%. Any failures need investigation.
Delivery ErrorsTemporary and permanent rejection reasonsIdentifies specific blocks and rate limits

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Common Gmail Blocks and Errors

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Temporary Blocks (4xx Errors)

Gmail throttles senders who exceed its internal rate limits or whose reputation dips. These are temporary and resolve automatically, but they signal a problem. 
421-4.7.28 Our system has detected an unusual rate of unsolicited mail
originating from your IP address.
Causes and fixes:
  • Volume spike — you sent significantly more email than usual. Reduce volume and ramp up gradually. See IP and Domain Warm-Up.
  • High spam complaint rate — check Google Postmaster Tools. Reduce complaints by improving list quality and making unsubscribe easy.
  • Sending to many invalid addresses — clean your list. See List Hygiene.
Lettr automatically handles 4xx retries. Temporary blocks typically resolve within a few hours. If they persist beyond 24 hours, review your sending practices.

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Permanent Blocks (5xx Errors)

Permanent rejections mean Gmail refused the message entirely. 
550-5.7.1 Our system has detected that this message is likely
unsolicited mail. To reduce the amount of spam sent to Gmail,
this message has been blocked.
Error CodeMeaningAction
550-5.7.1Message blocked as spamReview content, check authentication, verify reputation
550-5.7.26DMARC authentication failedFix DMARC alignment — ensure your From domain matches your DKIM signing domain
550-5.7.27SPF record missing or invalidAdd or correct your SPF record
550-5.1.1Recipient address doesn’t existRemove the address from your list
421-4.7.28Rate limited due to reputationReduce volume, improve engagement metrics

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DMARC Enforcement

Gmail strictly enforces DMARC policies. If your domain publishes p=reject or p=quarantine and authentication fails, Gmail will reject or spam-folder the message with no exceptions. Verify your DMARC alignment: 
# Check your DMARC record
dig TXT _dmarc.yourdomain.com +short

# Expected output (example)
"v=DMARC1; p=reject; rua=mailto:dmarc@yourdomain.com; adkim=r; aspf=r"
Ensure the domain in your From address matches the domain used for DKIM signing in Lettr. Check this on the Domains page.

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Gmail and Engagement Signals

Gmail weighs recipient engagement more heavily than most providers. Low engagement leads to progressively worse placement over time.

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Signals Gmail Tracks

  • Opens — recipients who regularly open your emails signal trust
  • Replies — direct replies are the strongest positive signal
  • Moves to spam — the strongest negative signal
  • Moves from spam to inbox — a strong positive signal
  • Deletes without opening — a weak negative signal
  • Stars and labels — positive engagement signals

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Improving Engagement

  1. Segment by engagement — send more frequently to engaged recipients and less to inactive ones.
  2. Re-engagement campaigns — before removing inactive subscribers, send a targeted re-engagement email. If they don’t respond, suppress them.
  3. Send at the right time — use Lettr’s analytics to identify when your recipients are most active.
  4. Relevant content — personalize emails based on recipient behavior and preferences. See Personalization and Dynamic Content.

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Diagnosing Gmail Issues Checklist

Most Gmail delivery problems are visible in Postmaster Tools. Check domain reputation, spam rate, and authentication dashboards before investigating anything else.
Send a test email to a Gmail account. Open the message, click the three-dot menu, and select Show original. Confirm dkim=pass, spf=pass, and dmarc=pass all appear in the Authentication-Results header.
Search your sending domain and IPs on MXToolbox. Being listed on Spamhaus, SORBS, or other major blocklists will cause Gmail to reject or spam-folder your messages.
If delivery degraded after a content change, compare the new email against the previous version. Look for newly added URL shorteners, excessive images, or spam trigger words.
A gradual increase in spam complaints — even if still below 0.3% — will eventually cause reputation damage. Address the trend early.
Gmail requires a functional List-Unsubscribe header with one-click support for bulk senders. Lettr adds this automatically, but verify it hasn’t been overridden via custom headers in the API.

Emails Landing in Spam

General spam placement diagnosis across all providers.

Google & Yahoo Requirements

Authentication and unsubscribe requirements for bulk senders.

Sending Reputation

Build and maintain a strong sender reputation.

Outlook Delivery Issues

Troubleshoot Microsoft-specific delivery problems.