SMTP Retries and Deferrals: Understanding Email Delays & How to Fix Them

What are SMTP retries and SMTP deferrals?

SMTP retries and deferrals are built-in mechanisms implemented to smoothen out temporary problems in email delivery. Retries and deferrals are both critical for maintaining email reliability and ensuring that messages are not lost forever when conditions are less than ideal.

• SMTP retries: If an email does not reach its destination on the first attempt, the sending server does not classify it as a permanent failure. Rather, it starts a round of retries or re-attempts to send the email after some fixed time.
• SMTP deferrals: If a receiving server is not ready to accept an email, it may provide a temporary error code, usually in the 4xx range. This means that although the email isn’t deliverable at that moment, it isn’t rejected permanently. The email is therefore queued for a retry. It essentially guarantees that transient failures will not result in communications being dropped.

How SMTP retries and deferrals work

Most servers’ internal mechanisms are not your concern because they fall into SMTP retry and deferral, which are the building blocks of how emails are delivered. The system is set up so they can try to send the email multiple times without giving up right away if the email doesn’t go through the first time. 

An overview of the SMTP conversation

An email’s journey starts with the SMTP conversation, a series of command and response exchanges between the sender and recipient servers:

• The process is initialized by the client sending commands like HELO or EHLO to introduce itself.
• This is then followed by the MAIL FROM command to set the sender, and the RCPT TO command to indicate the receiver.
• The real content of the message is provided using the DATA command, which is transmitted only after these initial handshakes.
• In this conversation, the receiving server analyzes the request it gets and communicates back with status codes.
• A 250 response code indicates a successful transaction. If the server receiving the request is facing a temporary issue (like overload, temporary unavailability or a momentary glitch), it might return the 4xx series response. This lets the sending server know that the failure is temporary, and it needs to try delivering again later.

🔖 Related Reading:

‘451 Temporary Local Problem’ SMTP Error in Emails: Causes and Fixes

Scheduling retries: timing and intervals

When an SMTP server receives a temporary failure code, it doesn’t immediately abandon the delivery attempt. Instead, it schedules a series of retries based on predefined timing intervals. Most commonly, the server uses an exponential backoff mechanism that progressively increases the delay between retries. For example, the first retry could fire after a few minutes, the next retry after a longer time, et cetera.

This approach ensures that the email is not lost in transit while also avoiding flooding the server with fast, repetitive tries. These intervals can often be configured by the administrator based on the nature of their network and the server capacity.

The role of deferrals in email queuing

Deferrals occur during the initial part of the email delivery process, when the receiving server tells the sender they can’t process the email right now. Instead of outright rejecting the email, the server defers it—placing the message into a queue to be retried at a later time. Normally, this response (something like a 421 error code) indicates to the sender that this is not a permanent report but rather a temporary state of unavailability.

The queuing system is very much an integral part of email traffic management, especially in environments in which the servers experience sporadic spikes in volume. This way, the system is more effective in managing temporary surges and reduces the chances of missing important communications, as emails are deferred rather than rejected right away. Eventually, servers try again and those emails get back in the queue to be sent again—only this time they finally get through.

Common causes of SMTP retries and deferrals

Server congestion and resource limitations

• When a mail server receives a high volume of emails it can become overwhelmed. The high volume can be due to peak traffic hours or an unexpected surge. Adding to this challenge, the hardware resources (CPU, memory, bandwidth, etc.) are often limited.
• The server attempts to process the backlog, which can lead to temporary deferrals. This results in the server retrying emails until the load is reduced or new resources are available.

Greylisting and spam prevention techniques

• Greylisting is a common spam prevention strategy where incoming emails from unfamiliar senders are initially deferred.
• The recipient server temporarily rejects the email with the expectation that legitimate mail servers will retry sending the message after a short delay.
• While effective against spam, this technique can inadvertently introduce delays for genuine emails. The temporary deferral inherent in greylisting is a balancing act between security and timely email delivery.

Network and connectivity issues

• Intermittent network or connectivity issues can disrupt the communication between sending and receiving servers. These interrupts could be transient network outages, high latency, and packet loss in the data transfer process.
• If the SMTP conversation is disrupted due to a network fluctuation, the server may respond with a 4xx temporary failure. Thus, the sending server needs to retry the email delivery after the network conditions get better.

Misconfigurations and Software Bugs

• Emails could be misrouted or rejected due to incorrect DNS settings or mail server configurations.
• Bugs in the email server software can lead to improper handling of the retry logic or deferral messages, causing delays.

Impact on email performance and deliverability

They overwhelm the mail server

Too many retries and queued deferrals can place an intense load on mail servers, consuming resources such as CPU, memory, and network bandwidth. When many emails are repeatedly retried, the accumulation of messages in the queue can lead to longer processing times and may even cause system slowdowns or temporary outages.

Example: Consider a scenario where a company launches a large-scale email campaign. Due to a temporary connectivity issue, a significant portion of emails receives a deferral response and is queued for retry. As these messages repeatedly re-enter the queue, the server becomes overwhelmed, slowing down the processing of both deferred emails and new incoming messages. This overload not only delays email delivery but may also cause subsequent campaigns to experience similar issues.

They impact the user experience

Email delays directly hit user engagement, which can lead to serious business impacts. And when the emails land late, this situation detracts from the timeliness of what the message is about—whether it’s an important alert, promotional offer, or other timely update. There is a time gap from the customer’s request to processing, which can cause them to feel disengaged and lead to the loss of opportunities and even the trust of customers.

Example: Picture a promotional offer sent out to customers that is time sensitive. Multiple deferrals on many emails could cause recipients to get the offer after it has expired. Not only does this weaken the effectiveness of the campaign, but it could also harm the perception of the company as unreliable leading to lower future engagement and sales.

Best practices to reduce email delays

• Optimize server resources: Your mail servers should able to handle high volume email traffic. This might include scaling hardware resources, utilizing load balancers, and tuning system settings to avoid overload during high hours.
  
• Optimize retry and deferral configurations: Set your SMTP server settings to fine-tune retry intervals and deferral timeouts. Designing an exponential backoff strategy can control the frequency of request retries and avoid hammering the server when it is temporarily down.
  
• Monitor email queues and performance metrics: Regular monitoring of email queues, bounce rates, and retry intervals is essential. This means you can prevent issues that slow down your site from becoming full-blown problems.
  
• Handle greylisting wisely: Greylisting is an excellent weapon against spam, but if set too aggressively, the delays can be crippling. You may want to fine-tune greylisting parameters or implement whitelisting for trusted senders, to prevent legitimate email from being intentionally held up.
  
• Implement dynamic queue management: Double down on time-sensitive or important emails and deprioritize lower impact messages. This ensures that crucial communication will be delivered awaiting the onslaught of email traffic.
  
• Regularly audit and update configurations: Keep your email server software up-to-date and conduct regular audits of your system settings. This proactive approach helps identify misconfigurations and software bugs before they can lead to significant delays.
  

Advanced strategies for improving deliverability

Improving email deliverability takes more than just a few configuration changes. It is a holistic approach that combines technical optimization, strategic segmentation, and constant monitoring. Adopting sophisticated techniques will allow you to keep a high sender reputation, secure inbox placement, and effectively manage the ebb and flow of email volume.

Dedicated IPs and sender reputation management

Using dedicated IP addresses isolates your sending reputation, allowing you to build and maintain trust with receiving servers. By having a consistent email cadence and tracking crucial metrics (bounce rates, spam complaints, etc.) you can manage your reputation.

Example: A marketing team that sets up a dedicated IP and takes the time to monitor performance metrics can proactively uncover sending trends to prevent spikes in spam complaints and see better placement metrics as a result.

Authentication protocols (SPF, DKIM, DMARC)

Proper email authentication is key to prevent spoofing and ensure legitimacy:

• SPF authenticates that emails come from authorized servers
• DKIM adds a verifiable digital signature to your messages
• DMARC enforces policies and enables reporting. These protocols work together to build credibility with ISPs.

Example: One such implementation could be a business that observes strong SPF, DKIM and DMARC records—reducing the chances of getting their emails marked as spam.

🔖 Related Reading: Why Do You Need to Configure SPF, DKIM, DMARC and How To Set Them

Engagement-based segmentation and content optimization

The relevance and quality of your email content also impacts deliverability. Segment your audience by engagement levels then tailor your messaging to the recipients’ interest. High engagement rates indicate to ISPs that your emails are valuable, improving your sender reputation.

Example: Targeting highly engaged users with customized content improves open rates and increases a good sending reputation, which ultimately has a positive overall effect on future deliverability.

Feedback loops and continuous monitoring

By building feedback loops with the largest ISPs, you receive real-time data on issues such as spam complaints and delivery failures. Constantly tracking delivery metrics—like bounce rates and engagement levels—enables you to identify and fix issues before they develop into bigger problems.

Example: An IT admin which uses feedback loop data can detect a spike in spam complaints and immediately adjust sending strategies to stop further sideline of emailing campaigns.

How to leverage Warmy for proactive measures against temporary email delivery failures

Warmy.io isn’t just an email warm-up tool—it’s a comprehensive, AI-driven solution that also helps tackle the challenges of SMTP retries and deferrals head-on. By focusing on enhancing email deliverability and optimizing sender reputation, Warmy.io helps prevent many of the issues that lead to temporary email delivery failures.

Automated gradual email warmup that mimics human-like interactions
Email warmup services reduces the risk of SMTP errors, and improves the number of emails sent out from a new or underused email address by gradually increasing the number of emails sent over time, while simulating natural email interactions. 

Warmy.io automates this process to help build a positive sender reputation and reduce the likelihood of deferrals triggered by spam filters.

Advanced seed list
The warmup process is complemented by Warmy.io’s seed list which contains genuine email addresses that act as seeds to mimic human engagement. Currently, there are seed lists for Gmail and Outlook—with Yahoo! coming soon.

Compared to traditional warming, these advanced seed lists enable real user behavior, ensuring emails are opened, scrolled, and clicked. Plus, if emails land in spam, these are manually removed and then marked as important to improve future deliverability.