At Netcore Cloud, we work hard to deliver your emails quickly and reliably—especially transactional ones like OTPs, order confirmations, and password resets. Sometimes, however, emails take longer than expected to reach the recipient's inbox. This is called delayed delivery.

Delayed delivery occurs when an email is not immediately rejected (no hard bounce) but is temporarily deferred, queued, or retried by the recipient's mail server or our own delivery system. These delays are almost always temporary and resolve on their own. They are frequently accompanied by soft bounce SMTP responses (4xx codes), which mean "try again later."


Common Causes of Delayed Delivery:

  1. Greylisting by Recipient Servers Many email providers (Gmail, Outlook, Yahoo, corporate domains) temporarily reject emails from senders they don't fully recognize yet. This is an anti-spam technique called greylisting. Legitimate mail servers retry automatically, and the email is usually accepted on the second or third attempt.
  2. Rate Limiting / Throttling by Recipient Servers Major providers impose invisible limits on how many emails they accept per hour or day from a single IP address or domain. If your sending volume exceeds these thresholds, the server defers acceptance until the rate drops.
  3. Recipient Server Overload or Temporary Issues The recipient's mail server may be experiencing high traffic, short-term resource constraints, maintenance, or processing delays.
  4. Content Scanning, Anti-Spam, or Virus Checks Emails containing attachments, complex HTML, many links, images, or elements that look potentially suspicious go through deeper scanning, which takes extra time.
  5. Large Message Size or Complex Attachments Heavier emails (large images, big attachments, or intricate formatting) are processed more slowly by both our servers and the recipient's.
  6. Network, DNS, or Connectivity Transient Issues Brief internet routing problems, slow DNS lookups for MX/SPF/DKIM records, or minor network interruptions between servers.
  7. Authentication Verification Delays Recipient servers perform additional checks on SPF, DKIM, and DMARC alignment. If DNS responses are slow, this step can add time.
  8. Queueing or Prioritization on Netcore Side During peak platform usage, emails may briefly queue. We give priority to transactional emails over marketing or promotional ones to ensure critical messages arrive faster.

Related SMTP Error Codes You Might See under Live feed section are as below,


These are the most common 4xx (soft/temporary) codes returned by recipient MX servers during delayed delivery attempts. They indicate the server is asking your mail server to retry later:

  • 421 – Service not available / Too many connections / Greylisted
  • 450 – Requested mail action not taken: mailbox unavailable / Temporary failure / Greylisted
  • 451 – Requested action aborted: local error in processing / Temporary failure
  • 452 – Requested action not taken: insufficient system storage / Mailbox full (temporary)
  • Other related codes: 421 4.7.0 (Temporary system problem), 450 4.7.1 (Greylisting in effect), 451 4.3.0 (Other mail system status), 451 4.4.0 (DNS lookup failure – temporary), 452 4.3.1 (Out of memory / system full)

Note: These 4xx codes are NOT permanent failures. They differ from 5xx hard bounce codes (e.g., 550, 554), which mean the email can never be delivered and the address should be suppressed.


What to Do to Minimize or Prevent Delays:

Here are practical steps you can take to reduce the frequency and duration of delayed deliveries:

  • Warm up new sending IPs and domains gradually — start with low daily volumes and slowly increase over days/weeks.
  • Spread large campaigns over time instead of sending high volumes in short bursts.
  • Classify emails correctly in the API — use the transactional category for urgent messages (OTPs, alerts, etc.) so they get priority.
  • Optimize email content and size — keep total size under 25MB, compress images/attachments, use links instead of heavy embeds, avoid spammy triggers (excessive caps, too many links, suspicious phrases).
  • Maintain excellent sender reputation — keep bounce rates below 2–5%, use double opt-in, remove inactive subscribers, encourage engagement, and always include a clear unsubscribe link.
  • Set up and verify strong email authentication — publish correct SPF, DKIM, and DMARC records (use our setup guides and verification tools).
  • Use dedicated IPs — if you send high volumes regularly, consider upgrading to a dedicated IP for more consistent throughput.
  • Schedule marketing/promotional sends during off-peak hours (avoid peak business hours in target regions).
  • Monitor your sending performance — regularly check our Latency Reports, bounce logs, event webhooks, and dashboard to spot patterns early.


Netcore Cloud Retry Policy for Delays and Temporary Failures:


Our delivery system automatically handles temporary issues so you don't have to intervene:

  • Soft bounces (4xx codes) and delayed/queued emails are retried automatically for up to 5 days.
  • Retries follow an exponential backoff schedule starting with shorter intervals (e.g., minutes) and gradually increasing (hours to days) to give servers time to recover without being overwhelmed.
  • Transactional emails receive priority in both queuing and retry attempts for the fastest possible delivery.
  • If a temporary issue persists beyond the 5-day retry window, the system may eventually classify it as a hard bounce and suppress the address to protect your sender reputation.
  • Hard bounces (5xx codes) are never retried — the address is suppressed immediately.
  • All retry logic is managed server-side; no manual action is required from you.


Most delays are short and resolve automatically through retries. If you consistently see long delays for specific domains, campaigns, or high-volume sends, review your reports or reach out to our support team. We can analyze your account setup, sending patterns, and reputation to provide personalized recommendations.


We're committed to helping your emails arrive as quickly as possible!