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 RecipientThe email account that you tried to reach does not exist
Gmail has determined that the recipient address does not exist. This is a permanent failure and should be treated as a hard bounce.
How to Fix:
- ✓Remove this recipient from your mailing list immediately
- ✓Do not retry sending to this address
- ✓Double-check for typos or unnecessary spaces in the email address
- ✓Implement email validation before adding addresses to your list
452 4.2.2
Mailbox FullThe recipient's inbox is out of storage space
The recipient has exceeded their Gmail storage quota. This is a temporary condition that may resolve when the user clears space.
How to Fix:
- ✓Treat as a soft bounce
- ✓Retry sending over several days
- ✓Suppress after 3-5 consecutive failures over 4-6 weeks
- ✓Direct recipient to https://support.google.com/mail/?p=OverQuotaTemp
550 5.7.1
Low Domain ReputationMessage likely suspicious due to very low reputation of sending domain
Gmail has blocked this message because the sending domain has poor reputation or no sending history. This commonly affects new domains or domains with problematic sending patterns.
How to Fix:
- ✓Build domain reputation gradually with engaged recipients
- ✓Start with low volume and increase slowly
- ✓Ensure proper authentication (SPF, DKIM, DMARC)
- ✓Monitor domain reputation in Google Postmaster Tools
550 5.7.1
Unsolicited MailMessage detected as likely unsolicited mail
Gmail has identified this message as spam based on content, user complaints, or sending patterns. Messages previously marked as spam will likely be blocked.
How to Fix:
- ✓Review email content for spam triggers
- ✓Ensure you have explicit permission to email recipients
- ✓Check spam complaint rates in Google Postmaster Tools
- ✓Improve list hygiene and remove unengaged users
421 4.7.28
URL Rate LimitUnusual rate of unsolicited mail containing your URL
Gmail has detected spam containing URLs from your domain. Your domain's URLs are being rate limited to protect users.
How to Fix:
- ✓Review all emails containing your URLs
- ✓Check if your domain is being used in spam campaigns
- ✓Avoid URL shorteners or redirects
- ✓Monitor URL reputation and remove problematic links
421 4.7.28
DKIM Domain Rate LimitUnusual rate of unsolicited mail from your DKIM domain
Gmail has rate limited your DKIM signing domain due to detected spam patterns. This affects all mail signed with this DKIM domain.
How to Fix:
- ✓Review all senders using your DKIM domain
- ✓Check for compromised accounts or unauthorized usage
- ✓Monitor DKIM domain reputation in Postmaster Tools
- ✓Consider separate DKIM domains for different mail streams
421 4.7.28
SPF Domain Rate LimitUnusual rate of unsolicited mail from your SPF domain
Gmail has rate limited your SPF authorized domain due to spam detection. This affects all IPs authorized in your SPF record.
How to Fix:
- ✓Audit all IPs in your SPF record
- ✓Remove unnecessary IP authorizations
- ✓Check for compromised systems sending mail
- ✓Use SPF record optimization to minimize inclusions
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:[email protected]>
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.