KumoMTA Case Study – AWeber
How AWeber Achieved Transparent, Scalable Email Delivery with KumoMTA
Founded in 1998 by CEO Tom Kulzer, AWeber is a leading email marketing platform that has served over one million entrepreneurs and small businesses over its 25+ year history. Based in Pennsylvania, AWeber provides powerfully simple email marketing solutions with award-winning 24/7 customer support, helping businesses connect with their prospects and customers through automated email campaigns, newsletters and marketing automation tools.
As one of the pioneers in the email marketing industry, AWeber processes massive volumes of email daily for its diverse customer base. This scale requires robust, reliable mail transfer agent (MTA) infrastructure that can handle high-velocity email delivery while maintaining excellent deliverability rates. When the firm’s long-standing MTA solution began showing its age, AWeber needed a modern alternative that could support its architectural philosophy and future growth plans.
The Challenge – Delivering Business Value in an Evolving Marketplace
AWeber had been using Momentum as its MTA for many years, a solution that initially served them well but gradually became problematic as their business evolved and the email landscape changed. The challenges they faced with Momentum were multifaceted, touching on competitive concerns, architectural limitations and declining support quality.
From a strategic perspective, AWeber was increasingly uncomfortable with potential competitive conflicts. The company that had originally developed Momentum as an on-premises enterprise software offering had launched a cloud version with some automated email marketing capabilities that duplicated some of AWeber’s core offerings. As CEO Tom Kulzer explained, “At a certain point there becomes a competitive nature to what the MTA provider is offering. We were getting into a potential dynamic where AWeber was essentially funding our future competitor. We’d long had success taking the approach that our business logic, sitting on top of an MTA functioning as a last-mile delivery pipe, was the best way to deliver value to our customers. Getting into a situation where we’d have vendor lock-in with a potential competitor was something to avoid.”
Further, the architectural limitations of Momentum became increasingly problematic as email deliverability requirements evolved. As Gavin Roy, AWeber’s technical lead put it, “because the platform lacked support for individual sender queues, if one user had an issue, all the users on those IPs could have an issue. It’s a common drawback to shared IP infrastructure, but it became particularly concerning as domain reputation became more critical in email delivery over the past decade.” AWeber needed the ability to isolate problematic senders without affecting their entire customer base.
When the original developer of Momentum got acquired in 2021, what followed was several years of declining product advancement and slipping technical support. “Staff just became less knowledgeable as more experienced engineers moved on,” said Roy. When a technical issue led to a fairly lengthy service outage, the AWeber team started looking at alternatives. Here is when early discussions with KumoMTA began to take on a new urgency.
About KumoMTA
KumoMTA is the open-source email platform specifically designed for today’s high-volume senders. Among its key attributes:
The software is free to use, review, and modify as the user sees fit—no license fees.
Provides industry-leading performance—capable of saturating physical hardware—while ensuring that messages are safely queued to prevent loss or double-sending.
Delivers full real-time integration capabilities for configuration, message manipulation, and routing via its built-in policy engine and data source access.
Fully integrates into your DevOps environment, with a containers, and native support for both SOCKS5 and HAProxy forward proxies.
Backed by a 24/7 response SLA available directly from the team that built it.
The Solution – A New Approach to Last-Mile Delivery
AWeber’s journey to KumoMTA began well before their Momentum crisis, driven by the company’s philosophy of maintaining architectural independence and their growing frustration with existing limitations. The evaluation process took three to six months as the team weighed their options, including the possibility of building their own MTA solution in-house. KumoMTA stood out for several compelling reasons, with its open-source nature being a critical factor.
“The fact that it was open source made a purchase decision to go with KumoMTA much easier,” observed Kulzer. “Worst case scenario, if they fail and it’s not a viable business, we’ve still got the source code and we can still continue to develop it ourselves.” This approach aligned perfectly with AWeber’s philosophy of avoiding vendor lock-in while providing a safety net for their infrastructure investment.
The KumoMTA technical architecture addressed many of AWeber’s pain points. Unlike Momentum’s rigid, per-machine licensing model, KumoMTA supports modern deployment patterns that would enable ephemeral worker nodes to scale up on demand. This architectural flexibility meant AWeber could eventually transition to “a more cloud native type of application running in Kubernetes and with scalable workflows,” said Roy.
KumoMTA’s integration with RabbitMQ was another significant advantage. “We used RabbitMQ internally already,” shared Kulzer. “So instead of the clunky logging we had with the old MTA, we could immediately incorporate event-based logging. If an event happens, it’s in our database within a second or two, a dramatic improvement over Momentum’s hourly batch updates.”
The queuing mechanism in KumoMTA represented a fundamental improvement over Momentum as well. “The way that Kumo does queuing is completely different from the way Momentum worked,” explained Roy. “Kumo does it the way that a modern MTA needs to work to deliver better performance and more granular control over email delivery.”
Perhaps most importantly, KumoMTA’s transparent development model aligned with AWeber’s engineering culture. The open-source codebase meant they could see exactly what was happening under the hood, understand changes to the platform before deploying them, and even contribute improvements back to the platform.
The Results – Transparency and Development Velocity
The migration to KumoMTA delivered significant improvements across multiple dimensions of AWeber’s email infrastructure, from operational efficiency to engineering productivity and system reliability. The most immediate benefit was unprecedented visibility into their email infrastructure. “Ultimately, it kind of boils down to transparency,” explained Roy. “MTAs like Momentum take a black-box approach. When a new version got released, we’d have little to no visibility into what changed. With KumoMTA’s open-source model, we could see the new elements as code that had been rolled into the release. As such, we could test it internally before we graduated it to production.”
This transparency dramatically accelerated AWeber’s testing and deployment cycles. “Because we could see exactly what changed in each release,” explained Roy, “the internal test / graduate to production process was really short. We’d know whether changes were minor logging updates or fundamental modifications that required extensive testing—an important distinction.”
Collaborative Support Model
The relationship with KumoMTA’s team felt fundamentally different from traditional vendor support. Said Roy: “It just felt like we had additional engineers on our team, with real-time communication through Slack and direct visibility into GitHub pull requests. When issues arose, we could have real-time communication with them about it. We would get to see the GitHub pull request kind of correlate and say, ‘oh look, we think it was here and here’s what they think fixed it.”
This collaborative approach proved crucial during the initial production stabilization period. “There was a production baking period, for lack of a better way to put it, of about three months” where AWeber and the KumoMTA team worked together to identify and resolve issues in real-time, explained Kulzer. Importantly, “there was never anything broken or underperforming that affected customers in a particular negative way. Actually, most issues became optimization opportunities rather than critical bugs.”
Architectural Flexibility
While the AWeber team initially deployed KumoMTA in a similar configuration to Momentum due to their short migration timeline, the platform’s architecture opened up new possibilities. They recognized that KumoMTA enables them to “foundationally, architecturally deploy the MTA in a different way, with the ability to scale up ephemeral worker nodes on demand,” explained Roy. This shift from “pets versus cattle” infrastructure philosophy meant they could eventually transition to elastic scaling models that automatically adjust capacity based on load—something that was impossible with Momentum’s machine-specific licensing model.
Peace of Mind
Perhaps most importantly, the migration eliminated a major source of operational stress. “I don’t wake up in the morning thinking about an MTA, I don’t go to bed and close my eyes thinking about an MTA,” reflected Kulzer. The platform now “does what it needs to do” reliably, allowing AWeber’s engineering team to focus on their core business rather than infrastructure concerns.
The stability and transparency of KumoMTA has transformed what was once a source of anxiety into a reliable foundation for AWeber’s email delivery operations. With features that meet the firm's current needs and a roadmap focused on performance optimization and cost reduction, AWeber has found a long-term MTA solution that aligns with both its technical requirements and business philosophy.
The success of the KumoMTA implementation demonstrates how the right infrastructure choice can transform operational challenges into competitive advantages, providing not just technical capabilities but strategic flexibility for future growth.
John Pinson has worked with members of the KumoMTA team for over 15 years. Over a long career in tech content marketing, he has written for brands including Google, Microsoft, Palo Alto Networks and many other industry leaders.
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