Is it better for ESPs to have IPs on different networks or on the same/adjacent network?

For ESPs, a single, larger contiguous block like a /22 is generally better than multiple smaller blocks. Larger blocks are perceived as more legitimate by ISPs and simplify management. Smaller, scattered blocks can be mistaken for snowshoe spamming, negatively impacting deliverability.

Why is IP warming important for new or unused IP addresses?

IP warming is a crucial process where you gradually increase the volume of emails sent from a new IP address over time. This helps build a positive sender reputation with ISPs. Without proper warming, ISPs may view sudden spikes in volume from new IPs as suspicious, leading to emails being deferred or sent to the spam folder. You can learn more about IP warming strategies to avoid these issues.

How many IPs should an ESP use for a daily volume of 500,000 emails to major ISPs?

If you're sending 500,000 emails daily to a major ISP like Yahoo or AOL, a single well-warmed dedicated IP or possibly two should be sufficient. Using a high number of IPs (e.g., 10 or 50) for this volume can be counterproductive, as it may mimic snowshoe spamming tactics and negatively affect your reputation. The key is to manage the optimal email volume based on your sender reputation, not just by adding more IPs.

What does it mean when MTAs go into "backoff mode" with deferral messages?

The error "Messages temporarily deferred due to unexpected volume or user complaints" indicates that ISPs perceive your mail as unwanted, often due to high spam complaints, even at low volumes. This is a reputation issue, not a technical one. The solution involves improving email content quality, ensuring list hygiene, and addressing why recipients are marking your emails as spam, rather than trying to fix it by adding more IPs. More information on why emails go to spam is available.

What is the best IP network configuration and email volume strategy for email service providers to avoid deliverability issues? - Sender reputation - Email deliverability - Knowledge base

Navigating the complexities of IP network configuration and email volume can be a daunting task for email service providers. The decisions made in these areas directly impact deliverability, influencing whether emails land in the inbox or are diverted to spam folders. Getting this right is fundamental to maintaining a strong sender reputation and ensuring reliable service for our clients.

Our goal is always to maximize inbox placement, which means understanding the intricate balance between IP address types, volume scaling, and reputation management. It is not simply about sending emails, but about sending them effectively and consistently.

In this article, I will share insights into the best practices for structuring your IP network and developing a robust email volume strategy to preempt deliverability issues. This includes examining the nuances of dedicated versus shared IPs, the importance of proper IP warming, and how to respond to common challenges like blocklisting and temporary deferrals.

Choosing your IP allocation

The choice of IP address configuration profoundly affects email deliverability. ESPs often face the decision between obtaining larger, contiguous IP blocks like a /22, or smaller, more fragmented blocks such as multiple /24s or even /27s. While it might seem like more IPs are always better, the structure of these blocks can significantly influence how recipient servers perceive your sending practices.

A single /22 IP block, which contains 1024 IP addresses, is generally preferred over a collection of smaller, scattered blocks. This preference stems from the fact that contiguous blocks appear more legitimate and less like the patterns associated with malicious senders. ISPs often apply reputation judgments to entire IP ranges, meaning that if one IP in a small, disconnected block gets a bad reputation, the entire block might be affected.

Conversely, using many small, scattered IP blocks, like 32 separate /27s, can unfortunately make your traffic resemble snowshoe spamming. This is a technique where spammers rapidly cycle through numerous IP addresses to evade filters. Even if your intentions are legitimate, this configuration can inadvertently trigger spam flags and lead to your IPs being blocklisted, damaging your sender reputation.

IP block size

Contiguous range: A single /22 block is perceived as more legitimate and stable by ISPs. It signifies a dedicated and organized sending infrastructure.
Simplified management: Less overhead for DNS records and IP management compared to many smaller blocks.

Deliverability impact

Better reputation: Less likely to be flagged as suspicious behavior like snowshoe spamming.
Stable trust: Allows for consistent IP warming and reputation building.

IP block size

Scattered ranges: Multiple /24s or /27s can appear disjointed, making it harder for ISPs to assess the overall sending pattern.
Increased complexity: More individual IPs mean more DNS records to manage and monitor.

Deliverability impact

Snowshoe risk: Can be mistaken for spamming tactics due to the distributed nature of sending.
Inconsistent reputation: Reputation building is fragmented across many small ranges, potentially leading to more frequent blocks (or blocklists) on individual IPs.

For ESPs, a single, larger contiguous block like a /22 offers significant advantages in terms of maintainability and reputation. It reduces the overhead of managing numerous smaller IP ranges and presents a more consistent sending profile to recipient servers, which can be crucial for avoiding deliverability issues.

The role of IP reputation and warming

IP reputation is the cornerstone of email deliverability. Every IP address has a reputation score that ISPs use to determine if incoming mail is legitimate or spam. This score is built over time based on sending behavior, including volume, bounce rates, complaint rates, and engagement. Starting with a new IP address, especially one that has been dormant, requires a careful IP warming process. Ignoring this can quickly lead to deliverability problems.

A common issue arises when ESPs acquire IP blocks that have been unused for extended periods. Even if these IPs were once clean, their dormancy can lead to them being categorized as suspicious once mail flow begins. This is why you might encounter messages like, "Please contact your Internet service provider since part of their network is on our block list," even when the IPs are directly assigned. This message is typical from

Microsoft and indicates that a portion of the network is already on an internal (or external) blacklist or blocklist due to past activity, or lack thereof.

Understanding IP blocklisting

When you encounter a blocklist (or blacklist) message, especially from major ISPs, it often means that the IP block, or a portion of it, has a pre-existing negative reputation. This can happen if previous owners of the IP space engaged in undesirable sending practices, or if the IPs were simply left fallow and picked up by spam traps or other internal filtering mechanisms.

Resolving this requires contacting the ISP directly to explain your situation and request delisting. It is not necessarily a sign that your current sending practices are at fault, but rather a reflection of the IP's history. Consistently applying best practices for IP warming and maintaining a clean list is essential to rebuild and protect the reputation of these IPs.

Even with perfectly configured IPs, an improper volume strategy can undermine all your efforts. Sending too much email too quickly, or with inconsistent volume, can trigger spam filters. ISPs like Google and Yahoo (AOL) closely monitor sending patterns. If your daily volume fluctuates wildly or suddenly spikes without adequate IP warming, your emails are likely to be deferred or sent to spam.

Email volume strategy

The optimal email volume per IP address isn't a fixed number, but rather a dynamic threshold determined by various factors, including the IP's reputation, subscriber engagement, and the recipient ISP's policies. For example, if you need to send 500,000 emails daily to Yahoo/AOL, a single IP or perhaps two dedicated IPs, if properly warmed, should suffice. Using ten or fifty IPs for this volume can make your sending look like snowshoe spamming, actively harming your deliverability.

When Mail Transfer Agents (MTAs) enter a "backoff mode" and defer messages with errors like "Messages temporarily deferred due to unexpected volume or user complaints," it's a clear sign that ISPs suspect unwanted mail. This isn't a technical configuration issue that can be solved by adding more IPs or tweaking settings like PMTA config. It's a reputation problem indicating that your subscribers are marking your emails as spam, even at low volumes. The solution lies in addressing the root cause, which is usually content relevance or list quality.

For ESPs, managing complaint rates is crucial. Even a small percentage of spam complaints, such as exceeding 0.10% on a dedicated IP, can lead to reputation damage at both IP and domain levels. Yahoo (AOL) and

Gmailare particularly sensitive to these signals and will implement throttling or blocklisting if thresholds are crossed. These systems see the entire message content, including links and domains, enabling them to make informed decisions about whether to accept your mail.

Ultimately, the right volume strategy revolves around consistently sending desired mail to engaged recipients. If you're experiencing deferrals or blocks, the primary focus should shift from technical configurations to improving the quality and relevance of the email content and ensuring robust list hygiene practices. Adding more IPs will not resolve underlying issues related to unwanted mail, and can even exacerbate them by further fragmenting your sending reputation.

Factor	Impact on deliverability	Recommendation
Sender reputation	High reputation allows for higher volumes; low reputation leads to throttling.	Focus on building a strong sender reputation over time.
IP warming	Essential for new IPs to gradually build trust with ISPs.	Implement a structured IP warming plan before increasing volume.
Complaint rates	High complaint rates (>0.1%) severely damage reputation and trigger blocks.	Maintain low complaint rates through content quality and list hygiene.
List hygiene	Sending to invalid or unengaged addresses increases bounce and complaint rates.	Regularly clean lists, remove unengaged users, and avoid purchased lists.
Email content	Spammy content or suspicious links can bypass IP reputation and trigger filters.	Ensure engaging, relevant content and proper authentication for your domains.

Views from the trenches

Best practices

Align IP allocations with sending volume to avoid looking like a snowshoe spammer.

Always warm up new IP addresses gradually, even if they come from a clean block.

Focus on content relevance and list hygiene to reduce spam complaints.

Common pitfalls

Using many small, scattered IP blocks which can lead to misidentification as snowshoe spamming.

Failing to warm up unused IP addresses, resulting in immediate blocks upon activation.

Attempting to solve deliverability issues solely by increasing the number of IPs.

Expert tips

"Many backoff modes are overly aggressive. You might consider adjusting your MTA settings to not trigger on the very first TS04 error, but always remember the true solution involves stopping the underlying cause of those errors."

"If you have older IP blocks that haven't been used, be aware that Microsoft, for example, might have them on a blocklist. You'll need to open a ticket with them and explain your situation."

"A good IP block is a /22. Anything smaller than a /24, such as 32 /27s, significantly increases maintenance overhead and can negatively impact deliverability perception."

Expert view

Expert from Email Geeks says a single /22 is much better than many smaller blocks like 32 /27s because it reduces maintenance overhead and is less likely to break something if other deliverability practices are correct.

2023-05-01 - Email Geeks

Expert view

Expert from Email Geeks says that if an ESP's traffic consists of multiple scattered /27s, it will likely be assumed to be snowshoe spamming based solely on DNS data, even if it's not the case.

2023-05-01 - Email Geeks

Summary and final thoughts

Achieving optimal deliverability as an ESP hinges on a holistic approach that integrates intelligent IP network configuration with a well-managed email volume strategy. It's not just about the raw sending power, but the reputation and trust you build with recipient ISPs over time.

Prioritizing larger, contiguous IP blocks like a /22, meticulously warming new or dormant IPs, and maintaining a consistent, appropriate sending volume are critical. Remember, messages like "temporarily deferred due to unexpected volume or user complaints" are not technical glitches but clear signals of reputation issues stemming from unwanted mail. The real fix lies in improving content relevance, ensuring list hygiene, and reducing spam complaints. By focusing on these core elements, ESPs can ensure their emails consistently reach the inbox and avoid costly deliverability setbacks.

Ultimately, deliverability is a continuous process of monitoring, adapting, and refining your sending practices. By embracing these best practices, ESPs can build and maintain the robust sender reputation necessary for sustained email marketing success.