Gmail SMTP Error Codes
Master Gmail email delivery with our comprehensive guide to SMTP errors, reputation management, and Google's strict authentication requirements.
🚨 Critical: February 1, 2024 Requirements
Senders sending more than 5,000 messages per day to Gmail must meet these requirements:
⚠️ Non-compliance may result in messages being blocked or marked as spam.
📧 About Gmail
Platform Overview
- • Launched April 1, 2004 (invitation-only)
- • Became publicly available in 2007
- • World's largest email service
- • Advanced spam filtering with AI
- • Strict authentication requirements
Unique Features
- • Google Postmaster Tools for monitoring
- • Aggregate FBL (not individual)
- • Dynamic connection limits by reputation
- • Internal filters (may incorporate Spamhaus data)
🛡️ Gmail's Anti-Spam Approach
Google doesn't publicly disclose their blocklist approach. Gmail relies primarily on its own internal filtering system. While they don't overtly rely on public blocklists, there's a possibility they incorporate data from reliable sources like Spamhaus in certain ways.
This is why monitoring your reputation in Google Postmaster Tools is crucial - it's your primary visibility into how Gmail views your sending reputation.
📅 November 2023 SMTP Updates
Since November 15, 2023, Google has changed many of its SMTP responses. Starting November 22, 2023, the new 421 4.7.28 errors specifically target senders not meeting the upcoming February 2024 requirements.
⚠️ Important: These errors are early warnings for senders who don't respect the new Google Bulk Email Senders Guidelines that will be enforced on February 1, 2024.
Gmail SMTP Error Codes Explained
550-5.1.1
Invalid RecipientGmail could not deliver the message because the destination mailbox does not exist.
Typos, stale data, or deactivated Google Workspace accounts cause 5.1.1 failures. Treat them as hard bounces and stop retrying immediately.
How to Fix:
- ✓Remove the address from your mailing list after the first bounce.
- ✓Validate addresses at form submission and during imports.
- ✓Offer subscribers a self-service profile center to keep data current.
- ✓Alert customer success teams when key accounts bounce so they can confirm addresses.
550 5.1.0
Invalid RecipientGmail rejected the message because the user part of the address is unknown.
Workspace administrators can remove or rename users without warning. Gmail then returns 5.1.0. Continued retries harm your sender score.
How to Fix:
- ✓Flag the subscriber record and request an updated address.
- ✓Remove catch-all rules that mask user churn.
- ✓Implement bounce categorisation so user-unknown events are not retried.
- ✓Avoid sending follow-up campaigns to secondary aliases if the primary user fails.
550 5.2.1
Mailbox DisabledThe destination mailbox exists but is disabled or suspended.
Google Workspace admins can suspend accounts or place them on hold. Gmail responds with 5.2.1. The address may be reactivated later.
How to Fix:
- ✓Treat the event as a soft bounce and retry in your normal cadence.
- ✓Escalate to the account owner if the contact is business critical.
- ✓Suppress the address after repeated failures over several campaigns.
- ✓Log the event for customer success follow-up.
452 4.2.2
Mailbox FullThe recipient mailbox is out of storage space.
Gmail refuses new mail when the account hits its quota. It is a temporary condition that should clear once the user frees up storage.
How to Fix:
- ✓Retry delivery for a few days before suppressing the address.
- ✓Encourage contacts to upgrade or free space if the relationship warrants it.
- ✓Stop sending marketing drips to mailboxes that stay full for weeks.
- ✓Use engagement scoring to prioritise retries for VIP recipients.
421 4.7.0
Temporary FailureGmail encountered a temporary server issue and asked you to try again later.
421 4.7.0 is usually an infrastructure blip on Google’s side. Respect the retry window or Gmail may extend the deferral.
How to Fix:
- ✓Implement exponential backoff when retrying the delivery.
- ✓Avoid hammering the same connection with rapid-fire retries.
- ✓Monitor for patterns that might indicate a broader Google outage.
- ✓Keep the incident timeline for postmortems with stakeholders.
550 5.7.1
Domain ReputationGmail blocked the message because the sending domain has low reputation.
New domains, poor engagement, or recent spam complaints trigger Gmail’s domain reputation filters. The message is rejected outright.
How to Fix:
- ✓Warm new domains slowly, starting with long-term engaged users.
- ✓Monitor domain reputation charts inside Google Postmaster Tools.
- ✓Align SPF, DKIM, and DMARC so authentication is airtight.
- ✓Segment aggressive promotions onto hardened domains or subdomains.
550 5.7.1
Content / ConsentGmail believes the content or consent for this campaign is not valid.
If a message resembles spam or recipients repeatedly mark it as junk, Gmail issues 5.7.1 and blocks future attempts with the same payload.
How to Fix:
- ✓Review copy, subject lines, and link strategy for spam traits.
- ✓Remove purchased, rented, or scraped lists permanently.
- ✓Place unengaged Gmail recipients into reactivation journeys before sending full campaigns.
- ✓Add prominent, one-click unsubscribe tools to reduce complaint pressure.
550 5.7.26
Policy ViolationThe message fails Google’s bulk sender guidelines—commonly IPv6, DNS, or authentication requirements.
Gmail returns 5.7.26 when an IPv6 sender is missing PTR records, when SPF/DKIM are absent, or when one-click unsubscribe headers are missing.
How to Fix:
- ✓Publish forward and reverse DNS for each sending IP (especially IPv6).
- ✓Ensure SPF, DKIM, and DMARC all align with the visible From domain.
- ✓Add RFC 8058 one-click unsubscribe headers for promotional traffic.
- ✓Follow Google’s February 2024 checklist and document compliance.
535 5.7.1
Authentication FailureGmail rejected the login attempt because the credentials or authentication method are invalid.
Incorrect passwords, disabled Less Secure Apps, or missing OAuth tokens surface as 535 5.7.1 before any mail is accepted.
How to Fix:
- ✓Verify the SMTP username and password or refresh the OAuth token.
- ✓Enable SMTP Authentication for the account inside Google Workspace.
- ✓Use OAuth 2.0 for programmatic access instead of basic auth where possible.
- ✓Check that two-factor authentication is not blocking the connection.
421 4.7.28
URL ReputationGmail throttled the campaign because the URLs inside are appearing in spam.
When Gmail detects large volumes of spam pointing to your URLs, it slows or blocks other messages that reference the same links.
How to Fix:
- ✓Audit messages to ensure no compromised accounts are spamming your links.
- ✓Avoid URL shorteners or open redirects that mask the destination.
- ✓Rotate or retire URLs that appear on blocklists and remediate the cause.
- ✓Monitor third-party affiliates to ensure they are not abusing your brand.
421 4.7.28
Domain ReputationGmail rate limited mail signed with this DKIM domain after seeing abuse elsewhere.
The same DKIM domain can be used across multiple senders. If any stream misbehaves, Gmail slows every message signed with that domain.
How to Fix:
- ✓Review all platforms and brands using the DKIM domain for compromise.
- ✓Segment transactional and marketing streams onto separate DKIM domains.
- ✓Rotate signing keys when a partner or vendor no longer sends on your behalf.
- ✓Track DKIM domain reputation inside Google Postmaster Tools.
421 4.7.28
Domain ReputationGmail slowed mail because the SPF domain is authorising IPs that are sending spam.
Overly broad SPF records (e.g., “include:*”) can expose your domain to abuse. Gmail rate limits all IPs in the record when abuse is detected.
How to Fix:
- ✓Remove unused IP ranges and vendors from the SPF record.
- ✓Flatten nested SPF includes so you can audit each address space.
- ✓Create dedicated SPF records per business unit or use case.
- ✓Coordinate with partners to ensure their traffic meets Google standards.
Gmail Best Practices
Authentication Setup
- •Implement SPF, DKIM, and DMARC
- •Use Author Domain Signature
- •Ensure domain alignment in authentication
- •Avoid shared or URL shortening domains
- •Set up proper PTR records
Volume Management
- •Start with low volume for new IPs/domains
- •Gradually increase sending (warm-up)
- •Monitor reputation in Postmaster Tools closely
- •Consolidate Google Workspace domains in shared queue
List Hygiene
- •Keep spam complaints below 0.3%
- •Remove hard bounces immediately
- •Suppress inactive users regularly
- •Use double opt-in for quality
Compliance
- •Include List-Unsubscribe headers (not visible to recipients)
- •Support one-click unsubscribe
- •Add List-Unsubscribe-Post header
- •Provide visible unsubscribe link in message body
Technical Configuration
Gmail Connection Management
# Good reputation = unlimited connections
# Neutral reputation = start low, increase gradually
# Recommend: Author Domain Signature and domain alignment
# Must include both https:// and mailto://
# https:// link should not require further validation
List-Unsubscribe: <https://example.com/unsubscribe>, <mailto:unsubscribe@example.com>
List-Unsubscribe-Post: List-Unsubscribe=One-Click
# See: https://support.google.com/mail/answer/81126
gmail.com, googlemail.com, google.com → single queue/connection
Google Postmaster Tools
📊 Key Metrics
- • Domain & IP reputation scores
- • Spam rate percentage (keep < 0.3%)
- • Authentication results (SPF/DKIM/DMARC)
- • Encryption (TLS) usage
- • Delivery errors breakdown
🎯 Reputation Factors
- • User engagement (opens, clicks)
- • Spam complaints (aggregate)
- • List quality (bounce rates)
- • Authentication compliance
- • Sending patterns consistency
Note: Gmail provides aggregate feedback only, not individual complaint notifications. Monitor your metrics regularly at postmaster.google.com. Some monitoring tools like Postmastery integrate with Google Postmaster Tools for easier tracking.
IP & Domain Warming Strategy
📈 Volume Progression
| Day | Volume | Target |
|---|---|---|
| 1-3 | 50-100 | Most engaged users |
| 4-7 | 200-500 | Recently engaged |
| Week 2 | 1,000-2,500 | Active subscribers |
| Week 3+ | Double weekly | Gradual expansion |
⚡ Acceleration Tips
- Good reputation allows 1M+ messages/day per IP (as witnessed)
- Monitor Postmaster Tools daily during warming
- Pause if reputation drops below "Medium"
- Focus on engagement over volume initially
Gmail Resources & Support
Need Help Meeting Gmail's Requirements?
With Gmail's strict February 2024 requirements and complex reputation system, ensuring reliable delivery requires expertise. Let us help you implement proper authentication and monitoring.