SMTP Error Codes Database
Decode SMTP errors from major email providers. Understand what went wrong and learn how to fix delivery issues to reach the inbox.
Why SMTP Errors Matter
SMTP error codes are your roadmap to understanding email delivery failures. Each major email provider (Gmail, Yahoo, Outlook) has their own specific error codes and requirements. Understanding these codes helps you:
- Quickly diagnose delivery problems
- Implement the right fixes for each provider
- Improve overall deliverability rates
- Avoid permanent blocks and reputation damage
SMTP Errors by Email Provider
Currently covering 7 major email providers with more coming soon!
Yahoo Mail
Complete guide to Yahoo SMTP error codes including TSS04, TSS09, TSS11 deferrals, authentication failures, and policy violations. Learn how to resolve common Yahoo delivery issues.
Gmail
Comprehensive Gmail SMTP error codes including 421, 450, 451, 550, and 554 errors. Understand Google's spam filters and authentication requirements.
Office 365 / Outlook
Microsoft Office 365, Outlook, and Hotmail SMTP errors including reputation blocks, authentication failures, and tenant-specific issues.
Apple Mail (iCloud)
Apple Mail and iCloud SMTP error codes. Understand Apple's unique delivery requirements and authentication standards.
AOL Mail
AOL SMTP error codes and delivery guidelines. Learn about AOL's specific requirements and common rejection reasons.
Comcast / Xfinity
Comcast and Xfinity SMTP errors. Note: Comcast is migrating to Yahoo Mail infrastructure in 2025.
Laposte.net
France's largest free webmail provider. Learn about LPN error codes, BIMI support, and specific French email requirements.
SFR
Major French telecom operator covering sfr.fr, neuf.fr, numericable.fr and more. Understand SFR_IN error codes and reputation-based limits.
Orange
Master Orange.fr and Wanadoo.fr delivery. Understand OFR error codes, bilingual messages, Cloudmark blocks, and Signal Spam feedback loops for France's largest ISP.
Common SMTP Error Categories
4xx Errors (Temporary)
- 421: Service not available, try again later
- 450: Mailbox unavailable (busy)
- 451: Local error in processing
- 452: Insufficient system storage
5xx Errors (Permanent)
- 550: Mailbox unavailable
- 551: User not local
- 552: Storage allocation exceeded
- 553: Mailbox name not allowed
- 554: Transaction failed
Quick Tips for Resolving SMTP Errors
Authentication
Ensure SPF, DKIM, and DMARC are properly configured for your domain
Reputation
Monitor your sender reputation and remove invalid emails from your list
Rate Limiting
Respect provider limits and implement proper retry logic with backoff
Need Help Fixing SMTP Errors?
If you're experiencing persistent delivery issues or need expert guidance on improving your email infrastructure, we're here to help. Get personalized solutions for your specific SMTP challenges.