Of all the options for communicating digitally, few are more effective than email. Yet when it comes to emailing en masse, transmission limits not only occur but are beyond frustrating. For those looking to become senders, SMTP rate limiting and throttling are two default defenses set in place more often than not by receivers of the mail server to prevent abuse, spamming, and overwhelming the mail server. Yet while such protective measures are necessary for safety and security across the board, for those looking to become senders especially for those with larger campaigns they pose a challenge. In this article we’ll explore SMTP rate limiting and throttling and why you’re hitting a wall with bulk sends with larger campaigns.
Thus, by identifying what these limits and restrictions are, understanding how to prevent them is crucial for successful transmission and campaign effectiveness.
What Is SMTP Rate Limiting and Why Does It Exist?
SMTP rate limiting refers to the maximum number of messages a sender is allowed to send within a specific time frame. Mailbox providers frequently employ rate limiting to prevent spammers and negative actors from abusing the sending community. If you send to 5,000 recipients at once all at the same time from the same IP, the server that serves those 5,000 recipients might limit or reject any additional messages from going out to that many people at once. This doesn’t mean your message is spam, it means the server is protecting itself and managing its resources. SMTP error 451.432 often appears in such cases, indicating a temporary rate limit restriction that suggests slowing down the message delivery rather than stopping it entirely.
Rate limits exist per receiving domain, per time of day, per health of the server, and per sender’s reputation. What goes through for one provider at one time (Gmail) may get throttled or rejected when sent through another (Yahoo or Outlook). These limits change over time and are rarely broadcast, making successful delivery that much more challenging.
Understanding Throttling and Deferred Responses: SMTP Rate Limiting
Throttling is when a receiving email server attempts to control how many more emails it wants to receive in a given time frame typically, at busy times. Unlike a hard bounce, where the email never goes through, with throttling, the email does not go through at that particular moment, but it is asking the sending server to attempt resending at a later time. Throttling is often associated with certain SMTP error codes like “421 Too many connections” and “450 Requested action not taken: mailbox unavailable.” This indicates that the email is not bouncing back because it’s unwanted, but instead, the server cannot take it now. There’s too much going on; there’s no bandwidth at the moment, or perhaps the sending server/email’s reputation is in question.
Throttling is the protection of email servers against being overwhelmed with excess incoming emails and bad server performance as a consequence.
Additionally, it happens when suspicious sending activities are detected that could be considered excessive or spammy. When you’re sending too much too quickly, the receiving server will try to delay intake and send a message to the sender to try resending. These are all forms of communication that your sending volume is not appropriate and must be adjusted for better email deliverability standards. Throttling does not equal an opportunity for messages to be lost. Generally, the sending server puts delayed messages into a queue and attempts to resend at a specific time.
Number of Hours
This can continue for a certain number of hours and even days based on the service provider. While throttled email is possible to receive eventually, the issue is that it can be received far too late. Many campaigns are dependent on receiving emails at specific times: flash sales, limited time offers, even essential transactional emails (like order confirmations and password resets) and when the email does arrive, days later, it makes the time-sensitive message moot.
Furthermore, the customer experience and customer engagement can be negatively impacted when people anticipate information that either never comes or comes so late, it doesn’t help them once they contact customer support. Keep reading to explore SMTP rate limiting and throttling and why you’re hitting a wall with bulk sends with larger campaigns.
Throttling
Throttling on any level, high or low can be detrimental to a campaign. If emails are throttled, delayed, or slow, by the time they reach a recipient, they’re no longer interested in opening the email, or worse, they never see it. Throttled emails can lead to low open rates and low click rates, both of which are frustrating outcomes that contribute to a campaign’s failure.
For industries that rely on conversions ecommerce, financial services, anything with a timeline for deliverables this adds unnecessary stress from a time-sensitive situation that can cost a company money. Even worse, someone may not realize their domain is throttled until too late.
The best way to prevent throttling is to control your delivery. Throttling does not happen overnight when new domains take off; it should be ramped up slowly, sending should be consistent but not overwhelming at any specific time, and feedback loops should be created with mailbox providers to ensure desired sender reputation. The more a sender can prove they are legit and not sending spam, the better. Additionally, authentication with SPF, DKIM, and DMARC also helps reduce the chances of throttling, so this is an endeavor well worth the effort.
How Sending Behavior Triggers Throttling: SMTP Rate Limiting
You’re also throttled based upon your sending reputation. When a new domain or new IP spikes in volume out of nowhere, automated red flags are raised that it’s spamming. The same is true with “email blasts” if you take three weeks off and all of a sudden send a 100K recipient campaign you’re going to get throttled. Likewise, if you send frequently but always to unengaged recipients or if a campaign NEVER gets opened, you’re getting rate limits applied.
Thus, in order to not reach the thresholds, one must keep a consistent sending frequency, warm up new IPs and domains gradually, and NEVER use “email blasts” because such things oversaturate and overwhelm inboxes without segmentation or timing. ISPs want to work with those who are consistent, predictable and accountable.
The Role of Reputation in Rate Limits
Mailbox providers determine sender reputation based on engagement metrics, complaints, bounces, and authentication. For example, when you have a low sender reputation, almost guaranteed throttling occurs and the opposite for a good reputation. A good one can grant you higher thresholds. Keep in mind that throttling is associated with your sender IP and domain. If you’re changing providers or starting a new domain, however, your threshold may reset, and they’ll be on to you for potential throttling until they’ve established you with a new IP/domain/new domain.
Therefore, sender reputation is important because it can make the difference between delivery success and failure. Avoiding a poor sender reputation is partly in your control. You must create engaged lists, proper list hygiene, calls to engagement, and removal of those who do not want to receive mail and help mailbox providers understand your legitimate mailing needs with proper SPF, DKIM, and DMARC. Safe and trusted senders get low throttling and better deliverability.
Monitoring and Responding to SMTP Feedback
You’ll also notice rate limiting or throttling in your logs. Your mail server or email service platform will acknowledge the occurrence through particular SMTP response codes. These are NOT extras, erroneous notations. These are troubleshooting evaluations that show in the moment what mailbox providers think of your sending history. For instance, “421 Too many connections” and “451 Temporary local problem” are issued communication from the receiving server that a. you’re going too much too fast or b. their server is overloaded and cannot do what you’re requesting at this exact time.
Thus, paying attention to your logs when you can receive them and access them is vital to make sense of not only your email performance metrics but longer trends, too. For instance, if you experience delivery delays in your log, you can ascertain over time which ISPs and domains have this same problem over and over again. Fixing things sooner than later averts blocks and reputation issues. With this insight, you may decide not to send to a certain domain, you may ensure your sends go slower and farther apart, or you may review your segments to accommodate only those who engage the most right away.
421 Codes: SMTP Rate Limiting Throttling
Yet throttling isn’t always uniform. Some mailbox providers throttle during busy times across the board. Other times, they assess sending behavior and generate 421 codes. For example, if you check your error log and see you’ve received a ton of 421s from one domain but none from the rest, it may be that one domain is dynamically throttling you based on your sender score with that provider. In these cases, retrying the messages is not an option changing the sending frequency or warm-up may be necessary.
If you’re going through an Email Service Provider, make sure to leverage the dashboards and reporting features available to you. Most ESPs can visually represent your domain’s success of delivery and show which SMTP errors are most prevalent, even citing how many times something happened to provide you with insight for change. You can collect this data and avoid the guessing game moving forward.
Ultimately, success comes from understanding what these response codes mean. Consider them not as obstacles, but as guidance. If you adjust your sending habits in response, you’ll not only bypass throttling, but also demonstrate to mailbox providers that you care about their feedback and know what compliance standards require for better deliverability. You’ll be known as a reputable sender, increasing the likelihood that future emails won’t be stuck further on or denied later.
Best Practices for Managing Bulk Sends
The best way to avoid rate limits is by using a staggered sending approach. Rather than sending one campaign to your entire list, break it up and send to smaller groups over time to avoid detection. For example, send to your most engaged and responsive individuals first. This helps the first delivery of the campaign and subsequent ROI with the campaign since they’ll care about what’s being sent if they’re interested in you already.
Also, use warming for new IPs or domains. Start with extremely low volume on Day 1 and slowly increase the volume over the next few days or weeks. This allows for mailbox providers to recognize you as a legitimate sender who was just starting out and not someone looking to spam their users.
Finally, keep in touch with your ESP or deliverability team. These experts often have their own contacts within major mailbox providers and could advocate for you if you find yourself constantly being throttled despite your efforts.
Why Bulk Senders Must Stay Proactive
Whether you operate a bulk email marketing company, a bulk email publishing site, or an e-commerce bulk email transaction sender, know that bulk email delivery does not come easy. Rate limiting and throttling are not punishments, they’re safety precautions. They’re designed to protect the end users and the email universe as a whole. They protect against spammers, international server crashes, and ensure that end users are not overwhelmed with a plethora of useless mail. However, to those legitimately sending bulk emails bulk email newsletters, bulk email time-sensitive offers can appear annoying, frustrating, and opaque. After all, no one from the receiving end ever explains why they’re punishing you.
But in order to circumvent the annoying precautions and ensure your newsletters, offers, and announcements get delivered and read you need to recognize that bulk email sending is a partnership. A partnership with the mailbox provider just as much as your intended email audience. Mailbox providers pay attention to every email sent to someone new; they’re not just tracking engagement but also tracking volume and overall technical compliance. Thus, sending bulk emails without appropriate pacing and warming is just like sending without care which raises flags for these providers and, in response, blocking or delays, and sender reputation may be destroyed forever.
Long Term Sucess
The key to long-term success is deliverability as a proactive approach. Everything should be adjusted based on how domains react to your sending habits. If one ISP is restricting or delaying your delivery, you’ll want to decrease the amount sent, increase time gaps between sending, and/or only send to the most engaged on that domain. You’ll also want to monitor your sending reputation and the health of it via authentication settings like SPF, DKIM, and DMARC, having them in place, verifying blacklists, and keeping bounce rates low.
In addition, relying on what a subscriber tells you is not enough. SMTP response codes provide insight as to what mailbox providers think about your mail. A 421 connection timeout or a 451 unavailable mailbox they all provide insight and should be monitored on a daily, weekly, and monthly basis to review delivery patterns, bounce rates, and SMTP responses so that a trend does not become a bigger problem.
Last Words
Ultimately, the success behind bulk email delivery does not stem from how much email is sent, but rather, how effectively. Fostering a relationship of trust with mailbox providers means that sending predictable, ethical, and expected emails enhances deliverability rates, reduces throttles, and keeps an audience more engaged. From the realities of the constraints of the ecosystem, emerging awareness and sending within means not only champions the process, but it also champions what used to be viewed as constraints as empowering guidelines that protect all parties involved those who send and those who receive.
