What are expected delivery times for one-time password (OTP) emails and how can they be measured?
Michael Ko
Co-founder & CEO, Suped
Published 17 Apr 2025
Updated 18 Aug 2025
6 min read
When you're dealing with one-time password (OTP) emails, speed is crucial. Users expect to receive these codes almost instantly to complete a login or transaction. Delays, even minor ones, can lead to frustration, abandoned carts, and increased support inquiries. Understanding what constitutes an expected delivery time and how to accurately measure it is key to maintaining a smooth user experience and strong email deliverability.
Unlike SMS OTPs, which often arrive within seconds, email delivery involves many variables. While there isn't a universally recognized ISO standard for OTP email performance, industry consensus often points to a target of under 5 to 10 seconds for these critical messages.
My goal here is to help you define realistic expectations for OTP email delivery times and provide practical strategies for measurement and improvement. This is about ensuring your authentication emails arrive promptly and reliably, boosting both security and user satisfaction.
Expected delivery times
Optimal delivery times for OTP emails are typically very short, often within seconds. Many users expect to receive these codes almost instantaneously after requesting them. Industry data suggests that a significant percentage of users complete transactions more readily when OTPs arrive within a 10-second window, highlighting the importance of rapid delivery. This reinforces the need for email systems to be highly optimized for transactional email delivery.
However, email is not an instant messaging service. Various factors can influence the actual time it takes for an email to land in an inbox, including the sender's reputation, network latency, and recipient mailbox provider processing. For businesses, the target should be to minimize this latency as much as possible, as even a few extra seconds can impact user experience and security.
I often aim for a delivery time of under 5 seconds for OTP emails. While this can be challenging to achieve consistently across all mailbox providers, it serves as an aggressive benchmark to strive for. Some providers, like Autodesk, even provide their own best practices to help ensure seamless delivery of these critical authentication emails, indicating a shared industry goal for rapid receipt.
How to measure OTP email delivery times
Measuring email delivery time, especially for time-sensitive messages like OTPs, involves looking beyond simple send-and-receive logs. While your email service provider (ESP) will log when an email was sent and when it was accepted by the recipient's mail server, this doesn't always reflect when it actually landed in the user's inbox and was seen.
One effective method is to measure the time from when your system triggers the email send to when the user successfully enters the OTP. This captures the full user experience latency, including any delays at the recipient's end. Breaking this down by specific mailbox providers, such as Gmail, Outlook, or Yahoo, can reveal specific bottlenecks or issues.
Another method is to utilize tracking pixels within your OTP emails. When a user opens the email, the tracking pixel loads, providing a timestamp closer to the actual inbox arrival time. This is particularly useful for understanding the email deliverability and inbox placement rates for different providers and segments.
User experience measurement
This approach measures the total time from OTP request to successful entry. It's a pragmatic measure of user satisfaction.
Pro: Directly reflects the user's perceived waiting time.
Con: Can be influenced by user behavior and network conditions post-delivery.
Factors impacting delivery time
Several factors can significantly impact the speed at which your OTP emails reach the inbox. Understanding these elements is essential for diagnosing delays and optimizing your email sending infrastructure.
Your sender reputation is paramount. If your domain or IP address has a low reputation, mailbox providers may intentionally delay your emails (greylisting) or even blocklist (blacklist) them, leading to significant delays or non-delivery. Monitoring your reputation through tools like Google Postmaster Tools is crucial.
The infrastructure of your email service provider (ESP) also plays a vital role. A robust ESP will have optimized sending queues and a well-maintained network to ensure emails are dispatched efficiently. Additionally, proper email authentication, including SPF, DKIM, and DMARC, is fundamental for fast delivery, as it signals trustworthiness to receiving servers.
Factor
Impact on delivery time
Mitigation
Sender reputation
Low reputation can lead to deferrals or blocklisting, increasing delivery time.
Maintain low bounce rates and spam complaints. Monitor blocklists.
Mailbox provider processing
Inboxes might queue or delay messages for filtering and security scans.
Internet delays can add milliseconds to seconds to delivery.
Use a reputable ESP with globally optimized infrastructure.
Content quality
Spammy content can trigger filters and delay delivery.
Keep OTP email content minimal and plain text-focused.
Understanding limitations and optimizing
While striving for instant delivery, it's important to remember that email is not designed for real-time communication. Your influence over the final time-to-inbox is limited once the email leaves your ESP's servers, particularly concerning the internal processing queues of major mailbox providers.
Focus on the aspects you can control: maintaining a pristine sender reputation, ensuring your email authentication records are correctly configured, and choosing an ESP with a strong deliverability track record. If your overall deliverability rate is consistently high (e.g., above 98%), small fluctuations in OTP delivery times might not indicate a significant issue.
Email OTP vs. SMS OTP
Email OTPs offer a balance of security and cost-effectiveness. However, their delivery isn't always instant due to the complex nature of email routing and spam filtering. Users generally understand that emails can experience minor delays.
SMS OTP (left)
SMS is generally faster for OTP delivery, often arriving within 1-2 seconds. It bypasses email filters, providing a more direct route to the user's phone.
Pros: Near-instant delivery, high open rates.
Cons: Higher cost per message, potential for carrier issues, less reliable in some regions.
Advanced measurement and troubleshooting
Example log entriesplain
2024-03-15 10:30:01 SENT user@example.com
2024-03-15 10:30:03 RECEIVED user@example.com (image load)
Measuring time-to-inbox can be challenging without direct access to recipient server logs. Your ESP's detailed logs are your primary source of truth, showing when they successfully handed off the email to the recipient's mail server. This is referred to as email sending speed, not necessarily time to inbox.
For a deeper dive into actual inbox placement times, you might consider using internal testing and monitoring. Set up test accounts with major mailbox providers and track the precise timestamp when an OTP email arrives. This can help you identify if specific providers consistently experience longer delays.
Remember that consistent delivery to the inbox is more important than achieving an arbitrary low delivery time if it comes at the expense of deliverability. A slightly delayed but delivered email is far better than an instantly sent email that ends up in spam or a blocklist (blacklist).
Views from the trenches
Best practices
Maintain exceptional sender reputation by avoiding spam complaints and bounces.
Implement strong email authentication (SPF, DKIM, DMARC) to build trust.
Use dedicated IP addresses for transactional emails if sending high volumes.
Common pitfalls
Ignoring slow delivery times, leading to poor user experience and abandonment.
Failing to segment transactional emails from marketing emails, impacting reputation.
Not configuring email authentication correctly, causing delays or blocks.
Expert tips
Prioritize user-centric measurement, such as time from request to OTP entry, over server-side metrics.
Regularly test OTP delivery to major mailbox providers using internal accounts.
Consider a fallback mechanism like SMS OTP for critical scenarios or persistent email delays.
Expert view
Expert from Email Geeks says that email should not be treated as instant messaging, and delivery times are influenced by numerous factors.
2024-04-08 - Email Geeks
Expert view
Expert from Email Geeks recommends measuring the time from sending the mail to when the recipient enters the OTP, as this provides a more meaningful metric than simply time-to-deliver. They also suggest breaking down delays by mailbox provider.
2024-04-08 - Email Geeks
Final thoughts on OTP email delivery
Achieving fast and consistent OTP email delivery is critical for user experience and security. While there isn't a hard standard, aiming for delivery within 5-10 seconds is a practical goal. The key lies in understanding the complex factors that influence email routing and actively monitoring your performance.
By focusing on a strong sender reputation, correct email authentication, and detailed measurement that includes user-side actions, you can optimize your OTP email strategy. This proactive approach ensures your critical messages arrive when and where they're needed, building trust and enhancing your overall email program's effectiveness.
Autodesk, even provide their own best practices to help ensure seamless delivery of these critical authentication emails, indicating a shared industry goal for rapid receipt.
