Based on our email monitoring bounce report, below are some of the common reasons why emails bounce and solutions to the problems.
1. Recipient does not exist
Problem: The email address is invalid or misspelled, as indicated by various “user unknown” or “no such user” messages.
Solution: Use double opt‑in to confirm email addresses at sign‑up, and integrate a real‑time verification API at the point of collection. Immediately remove all hard bounces from your active list after the first occurrence.
2. Recipient’s inbox is full
Problem: The recipient’s mailbox has exceeded its storage quota and cannot accept new messages, shown by “over quota” or “mailbox full” errors.
Solution: This can happen even with a valid email during sign-up. To adress this issue, temporarily suppress these addresses and retry after a few days; if the problem persists remove the email from your email list.
3. Recipient account is inactive/disabled
Problem: The Yahoo, iCloud or Gmail account has been disabled, closed, or is no longer active, preventing delivery.
Solution: Remove addresses that have not opened or clicked in 6‑12 months. Monitor engagement metrics and suppress inactive subscribers before they become permanent failures.
4. Relay access denied
Problem: The sending server is not authorized to relay mail through the recipient’s mail server, often due to misconfigured SMTP relay settings or invalid credentials.
Solution: Verify that your sending IP is correctly registered in your ESP’s SMTP relay settings and that all relay credentials are up‑to‑date.
5. Sender not authorized/unauthorized
Problem: The sender’s IP, domain, or email address is not permitted to send to the recipient, either due to sender policy restrictions or lack of authentication.
Solution: Ensure that your sending domain is fully verified and that you are using a properly configured sending IP or you have setup SPF, DKIM, and DMARC.
6. Blocked as spam/suspicious
Problem: Gmail or other providers have detected spam‑like characteristics, low sender reputation, or unsolicited content and have rejected the message.
Solution: The fix to this issue is to maintain a high sender reputation by authenticating with SPF, DKIM, and DMARC; keep spam complaint rates below 0.1%; and use content best practices (clear subject lines, plain‑text alternatives, and a visible unsubscribe link). Warm up new IPs gradually. If you are using a shared ip, try switching to decicated one, it is about $4 a month.
7. DNS/MX record lookup failed
Problem: The recipient’s domain does not have a valid MX record, or the DNS query returned an error (NXDOMAIN), meaning the domain itself does not exist or has no mail server.
Solution: Run a domain validation check (e.g., MX lookup) during sign‑up and block submissions from domains that fail. Regularly clean your list by removing all addresses from domains that no longer resolve.
8. Rate limiting exceeded
Problem: The sender has sent too many messages within a short period, triggering temporary rate limits.
Solution: Some email providers, like Gmail, have a limit on the number of emails you can send per day. The solution here is to limit yourself to the allowed quota or switch to an email marketing platform that has high limit. Gmail is meant or personal communication not mass mailing.
9. Message expired/timeout
Problem: The delivery attempt expired because the message could not be delivered within the allowed time, often due to persistent temporary server issues.
Solution: Configure your sending systems with proper retry schedules (exponential backoff) and monitor transient error rates. If a domain consistently times out, consult your ESP or consider suppressing those addresses.
10. Invalid credentials/authentication failed
Problem: The username, password, or API key used to authenticate with the SMTP server is incorrect, expired, or rejected.
Solution: Test SMTP credentials in a staging environment before you begin sending marketing emails.
11. Policy‑Based Rejection (Group / Forwarding / Allow‑List Rules)
Problem: The recipient’s organization enforces policies that restrict external senders, block forwarding, or require the sender to be on a specific allow‑list.
Solution: Respect recipient‑level policies; if a particular domain or organization consistently rejects your mail, then remove user from your email list. That is why it is important to confirm user email to detect such emails at earlier stage.
12. Temporary system/network problems
Problem: The recipient’s mail server or intermediate relays are experiencing transient issues (e.g., resource exhaustion, internal errors).
Solution: classify them as soft bounces and try sending in the next newsletter . If it fails after three different newsletetrs, then remove them from your emails list.
13. Message non-compliant (missing headers)
Problem: The email is missing required RFC headers (e.g., From, Date, Message‑ID) or has malformed structure, causing the server to reject it outright.
Solution: Use a well‑tested email construction library or service that guarantees all mandatory headers are included and properly formatted. Validate your messages against RFC standards before sending.
14. DKIM / Authentication Failure
Problem: The email fails DKIM signature verification or other authentication checks (SPF alignment, DMARC), leading the receiver to distrust the sender.
Solution: Sign all outbound emails with a valid DKIM key, ensure SPF includes your sending IPs, and publish a DMARC policy (p=quarantine or p=reject) to enforce authentication. Test your setup with deliverability tools before each major campaign.
15. Sender IP / Domain Blacklisted
Problem: Your sending IP or domain appears on a public or private blocklist, causing direct rejection.
Solution: Monitor your IP reputation using services like MXToolbox or Google Postmaster Tools. If blacklisted, follow the delisting process immediately, and implement strict list hygiene to maintain a low complaint rate. If you are on shared hosting, consider purchasing a dedicated IP, as you have no control over other users’ sending behavior on the same IP.
