When an email fails to send or deliver, you may see a three-digit error code. These codes help identify what went wrong. Below is a guide to the most common codes, grouped by whether the issue originates with your email service provider (presumably Webnames.ca) or the recipient’s email service provider.
Errors Generated by the Sender’s Email Service Provider
Usually fixable by you or the Sender's mail provider (e.g., authentication, syntax errors).
Code | Meaning | Cause | Solution | Who Fixes It? |
421 | Service not available | Temporary server issue | Wait and retry | Sender/Webnames.ca |
450 | Mailbox unavailable | Recipient’s mailbox busy | Retry later | Sender |
451 | Processing error | Temporary server issue | Retry later | Sender/Webnames.ca |
452 | Insufficient storage | Server resources low | Retry later | Webnames.ca |
500 | Syntax error | Bad SMTP command | Check email client settings | Sender |
501 | Invalid parameters | Bad email address format | Correct address | Sender |
530 | Authentication required | Missing login | Enable SMTP authentication | Sender |
535 | Authentication failed | Wrong credentials | Update login details | Sender |
Errors Generated by the Recipient’s Email Service Provider
Recipient-side issues (mailbox full, policy blocks): Require recipient or their provider to resolve.
Common Deferral Codes and Meanings
Code | Meaning | Most Likely Cause | Solution | Who Fixes It? |
421 | Service not available | Recipient server temporarily unavailable | Wait and retry | Sender/Webnames.ca |
450 | Mailbox unavailable | Recipient mailbox busy or locked | Retry later | Sender/Webnames.ca |
451 | Processing error | Temporary server issue | Retry later | Sender/Webnames.ca |
452 | Insufficient system storage | Recipient or sender server low on resources | Retry later | Recipient or Webnames.ca |
4.4.x | Network or DNS timeout | Routing issues or slow response | Retry later |
|
Why Do Deferrals Happen?
Greylisting: Recipient server intentionally delays unknown senders to block spam.
Rate limiting: Too many emails sent too quickly.
Temporary overload: Server resources maxed out.
DNS issues: Slow or failed lookups for MX records.
What Can You Do Regarding Deferral Messages?
Usually nothing immediately—Webnames.ca will retry automatically.
If you see repeated deferrals for the same domain:
Contact Webnames.ca Support with the bounce details.
Ask the recipient to whitelist your domain or IP if greylisting is suspected.
Avoid sending large volumes of mail too quickly.
Common Error Codes and Meanings
Code | Meaning | Cause | Solution | Who Fixes It? |
550 | Mailbox unavailable | Address doesn’t exist | Verify recipient address | Sender |
551 | User not local | Wrong domain | Correct domain | Sender |
552 | Mailbox full | Recipient over quota | Ask recipient to clear space | Recipient |
553 | Invalid recipient address | Typo or blocked domain | Correct address | Sender |
554 | Transaction failed | Spam filter or policy block | Adjust content or authentication | Sender + Recipient |
Enhanced Status Codes
5.1.1 – Recipient address does not exist.
5.7.1 – Message rejected due to policy (e.g., spam score, authentication failure).
Best Practices
Double-check recipient addresses.
Ensure your email client uses correct SMTP settings.
Avoid spam-like content (excessive links, misleading subjects).
Contact Webnames.ca Support if errors persist.