What is SMTP delay?

SMTP (Simple Mail Transfer Protocol) is a standard protocol used for sending emails across the internet. It allows communication between mail servers to relay mail messages from senders to the recipients’ email providers.

What is SMTP delay?

When sending an email, your email client connects to your organization’s mail server using SMTP. The mail server then contacts the recipient’s mail server to deliver the email. This mail relay process happens very quickly, usually taking just a few seconds.

However, sometimes there can be a delay in email delivery when using SMTP. This is known as SMTP delay. Let’s take a look at what causes it and how it can be optimized.

What Causes SMTP Delay?

There are several potential causes of SMTP delay:

1. Network Connectivity Issues

Network problems like internet outages or congestion can affect SMTP connections between mail servers. This disrupts the quick relay process, leading to delayed email delivery.

2. Server Capacity Problems

If the sending or receiving mail server is experiencing high volumes or hardware issues, it may throttle SMTP connections. This slows down the transfer of emails.

3. Spam Filters

Security filters that scan emails for spam and malware can cause SMTP delay. More rigorous scanning takes more time and slows email delivery.

4. Greylisting

Some mail servers use greylisting to temporarily reject emails from unfamiliar sources. Legitimate emails will retry delivery per SMTP protocol, but spammers usually won’t retry. This technique can delay initial email delivery.

5. Overloaded Mailboxes

For recipient mailboxes near or over the storage limit, the mail server may throttle message delivery to that account. This trickles emails in slowly rather than overwhelming the account.

6. Maintenance or Outages

Routine maintenance or unexpected outages on mail servers can temporarily disrupt SMTP connections and delay email delivery.

How Long Does SMTP Delay Last?

SMTP delays are usually temporary and last from a few minutes up to several hours. Emails may take longer than normal to send or arrive but will eventually get delivered if no permanent failure occurs.

If an email is delayed more than 4-6 hours, there is likely a more serious underlying issue that needs addressed.

Troubleshooting SMTP Delay Issues

Here are some steps to identify and resolve excessive SMTP delays:

  • Check server status pages – Look for notices of known issues from your email provider or network operators. This can reveal temporary outages.
  • Try sending from alternate devices – If delay only happens on one device, the issue may be local. Trying other devices helps pinpoint the source.
  • Look for patterns – Note if delays only happen at certain times of day or with certain recipients. This can indicate peak load issues.
  • Review server logs – Your email administrators can check the mail server logs for error patterns related to connectivity or capacity.
  • Change SMTP port or server – Switching ports or servers for outgoing mail may route around a problem. Consult your email provider for options.
  • Contact your email provider – Open a support ticket if delays persist without explanation. The provider can investigate issues on their infrastructure.
  • Check reputation – Some recipient servers automatically delay mail from senders with poor reputation. This is difficult to resolve as a sender.

Best Practices to Optimize SMTP Delivery

Here are some tips to optimize your SMTP email delivery:

  • Maintain your IP reputation – Avoid getting blacklisted by sending wanted mail only and handling bounces/unsubscribe requests promptly. Use authentication methods like SPF, DKIM, and DMARC.
  • Limit attachment sizes – Large attachments can stall message acceptance and transmission. Keep attachments under 5 MB when possible.
  • Send during off-peak hours – If possible, avoid sending bulk email campaigns during peak traffic periods which vary by region.
  • Review copy limits – Most email providers have limits on recipient numbers per message. Break up mailing lists if needed to stay within limits.
  • Monitor server load – Watch queue lengths and server load. Throttle send rates if servers are overwhelmed.
  • Use dedicated IP – Dedicated sending IP addresses have better reputation than shared IPs. This avoids perceived spam patterns.
  • Check DNS settings – Ensure DNS records are properly configured for your domain, including reverse DNS. This helps with deliverability and spam filtering.

Solutions for Persistent SMTP Delay

If SMTP delays persist despite troubleshooting, here are some solutions to consider:

  • Switch email providers or mail transfer agents – Chronic network or capacity issues may require moving to a new provider.
  • Set up a relay server – An interim SMTP relay can redirect messages around downstream bottlenecks.
  • Rent dedicated equipment – For very high volume, renting your own dedicated mail server may be justified to get the capacity and control needed.
  • Consult an email deliverability expert – Experts can audit your domain reputation, server settings, email content, etc. and provide optimization recommendations.
  • Evaluate the recipient domain – Some recipient domains are notorious for poor deliverability. Switching to an alternative contact address may resolve issues.

With proper troubleshooting and optimization, SMTP delays can be minimized to maintain fast and reliable email delivery. But persistent issues may require more substantial infrastructure changes to fully resolve.


  1. What is a normal SMTP delay?
    In optimal conditions, SMTP transfer between servers takes just a few seconds. Delays under a minute are common. Anything over 5-10 minutes indicates a delay worth investigating.

  2. Why is Gmail delaying my emails?
    Gmail may throttle delivery to overloaded accounts, graylist new senders, or scan for spam which delays initial delivery. G Suite accounts tend to have priority.

  3. Why is my GoDaddy email delayed?
    GoDaddy requires SMTP authentication and enforces reputation policies. Try enabling SMTP authentication in your mail client or use the GoDaddy SMTP server rather than a 3rd party one.

  4. Why is my Outlook email delayed?
    Outlook uses Office 365 servers which can have delays during peak loads or maintenance periods. Check their service status page for known issues.

  5. Can I prioritize delayed emails?
    Not directly. Emails are queued by the receiving server based on reputation, load, and other factors. As the sender, you have no control over delivery order/priority.

  6. Is greylisting causing my delay?
    Possibly. Greylisting temporarily rejects unrecognized sender domains. Legitimate emails get re-sent per SMTP protocol while spammers rarely retry. First emails get delayed.

  7. Why does delay happen at certain times?
    If delays consistently happen during certain hours, it usually indicates peak traffic load issues on sender or recipient servers in that timezone.

  8. Why are emails delayed going to one domain?
    Some domains like Yahoo or AOL are known to aggressively filter emails. It’s also possible their infrastructure is outdated compared to more modern email services.

  9. Can I change the SMTP port to resolve delays?
    Maybe. Switching sender SMTP ports occasionally routes around bottlenecks, but not always. Port 25 and 587 are common defaults. Try alternating between them.

  10. How can I speed up Office 365 SMTP delivery?
    Make sure to authenticate using Office 365 SMTP rather than a 3rd party server. Check Office 365 service health status. Increase internet bandwidth if needed.

  11. What causes delay after clicking send?
    Local outages, WiFi issues, bandwidth limits, or computer problems can delay send-time. Also confirm the email client is configured to match your SMTP settings.

  12. Why does delay happen more from some devices?
    Shared public IPs or weaker internet connections on some devices can impact SMTP delivery. Switching to a device with a better IP reputation may help.

  13. Can my company’s firewall cause SMTP delays?
    Yes, security products like firewalls can scan SMTP traffic and introduce delays. Contact IT to see if firewall settings can be adjusted to prioritize SMTP throughput.

  14. Will switching email services fix delay issues?
    If delays trace back to your current provider’s infrastructure or policies, moving providers may resolve it. Evaluate new options carefully first.

  15. Can I report delayed spam or phishing emails?
    Yes, most email services allow you to report spam, phishing, or delayed emails so they can improve filtering. This helps strengthen security and deliverability.

  16. How can I verify my mail server is working properly?
    Use a tool like Telnet, Mail Tester, or mxtoolbox.com to confirm your SMTP server can connect, deliver test messages, and pass basic security validations for your domain.

  17. Can switching SMTP ports resolve hotmail delivery delays?
    It’s possible. Hotmail servers can be finicky. Try alternating between ports 25, 587, and 2525 to see if one provides better Hotmail deliverability than others.

  18. Why do I get delivery delays when emailing large companies?
    Large companies are common targets for spam and abuse. They may aggressively filter emails from unknown outside senders which can delay deliverability until you establish a reputation.

  19. How does batching emails affect delay?
    Sending emails in large batches can overwhelm recipient servers. Consider throttling batch sizes and adding slight delays between messages to improve deliverability.

  20. Can I change the message priority to expedite delayed delivery?
    No, RFC standard SMTP does not include any mechanism for senders to prioritize emails. The recipient mail server controls the delivery order and throttling based on its own policies.

Key Takeaways

  • SMTP delay refers to lags in email delivery due to various bottlenecks in the relay process between mail servers.
  • Network issues, server capacity, greylisting, spam filters, and maintenance outages are common causes of SMTP delay.
  • Troubleshooting involves checking server statuses, using alternate devices/servers, reviewing logs, and contacting providers.
  • Best practices include managing IP reputation, monitoring server load, and optimizing DNS settings to maximize email deliverability.
  • For persistent issues, changing email providers, adding a relay server, or consulting experts may be required.


SMTP delay can negatively impact business communications and productivity when employees do not receive emails in a timely manner. By understanding the common causes and troubleshooting options, SMTP delays can be minimized through proper optimization and server management. For ongoing delay issues, more advanced solutions may be needed to resolve entrenched infrastructure bottlenecks affecting the timely relay of SMTP email.


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