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
Actionable fixes for Yahoo Mail SMTP bounce codes, from temporary TS throttles to permanent TSS blocks and policy failures.
Gmail
Resolve Gmail SMTP bounce codes covering reputation, authentication, and Google’s February 2024 bulk sender requirements.
Apple Mail (iCloud)
Troubleshoot iCloud and Apple Mail SMTP bounces, including Proofpoint reputation blocks, DMARC failures, and Apple-specific rate limits.
Microsoft 365 / Outlook
Troubleshoot Exchange Online SMTP bounces for Outlook.com and Microsoft 365, including ATTR routing issues, tenant policies, and Spamhaus blocks.
Laposte.net
Fix Laposte.net (LPN) SMTP bounce codes covering rate limits, DNS issues, DMARC enforcement, and French anti-spam policies.
SFR
Decode SFR_IN SMTP codes for SFR, Numericable, and Neuf domains, including rate limits, spam suspensions, and French complaint policies.
Orange
Diagnose Orange.fr and Wanadoo.fr SMTP bounces, including OFR rate limits, Cloudmark IP blocks, SPF checks, and French ISP content rules.
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.
Popular SMTP Error Fixes
Quick access to the most frequently searched deliverability fixes across major providers.
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.