At Netcore Cloud, we strive to ensure your emails reach their intended recipients reliably. However, email delivery can sometimes face challenges due to factors on the sender's side, recipient's side, or in between. Non-delivery typically falls into categories like bounces (where the recipient server rejects the email outright), delays that lead to failures, or silent issues where the email is accepted but doesn't land in the inbox (e.g., spam filtering).


We encourage you to please login to your CPaaS panel and check the email delivery logs using Live feed section to identify the exact reason for non-delivery of emails.


Below, we'll explain the main causes of non-delivery in simple terms, grouped by type. We'll also cover common SMTP error codes (these are standard responses from recipient mail servers during delivery attempts) with explanations and what you can do. SMTP codes starting with 5xx are "hard bounces" (permanent failures—stop sending to that address), while 4xx are "soft bounces" (temporary—retries may work). If you're using our Email API, we handle retries automatically for soft bounces (up to 5 days) and suppress hard-bounced addresses to protect your reputation.


  1. Hard Bounces: Permanent Failures (Email Can't Be Delivered Now or Ever) These happen when the recipient server permanently rejects the email. Common SMTP errors include:

    • 550: Mailbox unavailable / User unknown / Does not exist Explanation: The email address doesn't exist, is invalid, or the domain is unreachable (e.g., expired or typo). 
    • 550 5.1.1: User unknown / No such user Explanation: The specific mailbox isn't found on the server (e.g., account deleted). 
    • 551: User not local; try forwarding address Explanation: The address is valid but not hosted there forwarding is needed (uncommon today). 
    • 552: Exceeded storage / Mailbox full Explanation: The recipient's inbox is over quota and can't accept more emails. 
    • 553: Mailbox name not allowed / Syntax error Explanation: The address format is invalid or blocked by rules. 
    • 554: Transaction failed / Delivery not authorized Explanation: Rejected due to policy, blacklisting, or content issues. 
    • 554 5.7.1: Policy violation / Relaying denied Explanation: Sender lacks permission or fails security checks. What to Do: Set up SPF, DKIM, and DMARC properly to verify your emails.
    • 550 5.7.1: Message rejected (spam/policy/DMARC fail) Explanation: Flagged as spam or authentication mismatch. 
    • 550 5.7.26: Unauthenticated email rejected Explanation: Strict DMARC policy blocks unauthenticated emails. 
    • 550 5.4.1: Recipient address rejected Explanation: Blocked by the recipient's server (e.g., due to your reputation).
    • 554 5.6.0: Message malformed / Invalid format Explanation: Email structure violates standards (e.g., bad headers or attachments). 

  2. Other hard bounce codes: 5.1.0 (general invalid address), 5.1.2 (bad syntax), 5.1.6 (mailbox moved), 5.2.0/5.2.1 (disabled/inactive), 5.2.2 (full inbox), 5.7.0/5.7.13 (policy/account issues).

    General Causes for Hard Bounces:

    • Invalid or outdated email addresses (e.g., from old lists).
    • Blacklisted sender IP/domain (due to past complaints or spam reports).
    • Authentication failures (missing SPF/DKIM/DMARC).
    • Policy violations at major providers like Gmail or Outlook.

  3. Soft Bounces: Temporary Failures (Delivery Might Work Later) These are short-term issues where the server asks to try again. If repeated, they can become non-deliveries.

    • 421: Service not available / Too many connections / Greylisted Explanation: Server is temporarily down, overloaded, or greylisting (delaying new senders to check for spam). 
    • 450: Mailbox unavailable / Temporary failure Explanation: Temporary block or busy mailbox. 
    • 451: Local processing error / Temporary failure Explanation: Server-side glitch (e.g., during virus scan). 
    • 452: Insufficient storage / Mailbox full (temporary) Explanation: Inbox full but likely to clear soon. 

  4. Other soft bounce codes: 421 4.7.0 (system problem), 450 4.7.1 (greylisting), 451 4.3.0/4.4.0 (undefined/DNS issue), 452 4.3.1 (out of memory).

    General Causes for Soft Bounces:

    • Greylisting or rate limiting by recipients (common for new IPs).
    • Temporary server outages or high traffic.
    • Transient network/DNS problems.
  5. Other Causes of Non-Delivery (Beyond SMTP Errors) Even if no bounce occurs, emails might not reach the inbox:
    • Spam Folder Routing: Flagged as spam due to poor reputation, spammy content (e.g., too many links, salesy language), or low engagement.
    • Silent Drops: Some servers accept but discard emails without notifying (e.g., due to blacklisting).
    • Recipient Blocks: User-marked as spam previously, or domain-level filters.
    • Sender Reputation Issues: High complaint rates, bounces, or unknown IPs.
    • Compliance Problems: Violating laws like CAN-SPAM or GDPR (e.g., no unsubscribe link).
    • Technical Setup Errors: Misconfigured domains or servers on your end.



Possible solutions for Hard bounce cases:


  1. Remove this address from your list. Double-check for typos and use email validation tools before sending.
  2. Suppress the address and clean your contact list regularly.
  3. Update to the suggested forward if provided; otherwise, remove it.
  4. The recipient needs to clear space; if it persists, treat as permanent and suppress.
  5. Fix any formatting issues in your data and remove invalid entries.
  6. Check if your IP/domain is blacklisted (use tools like MX Toolbox). Improve email content and authentication.
  7. What to Do: Review content for spammy elements and monitor your sender reputation.
  8. Ensure DMARC alignment and test your setup.
  9. Work on improving engagement rates and request delisting if needed.
  10. Test your emails for compliance and keep sizes reasonable (<25MB).


Possible solutions for Soft bounce cases:


  1. Retries may succeed; suppress if it persists beyond 5 days. 
  2. Automatic retries handle this.
  3. Wait for retries; if frequent, check your send volume. 
  4. Our system retries automatically usually resolves in minutes to hours.
  5. Maintain low bounce/complaint rates (<2%), use double opt-in, personalize content, and track metrics via our dashboard. Tools like Google Postmaster can help monitor reputation.