Author: Roalt Zijlstra

I have been working in the email marketing industry for over 15 years and I have installed and configured most well known mass-mailers in market. Starting off my professional journey as an Open Source solutions engineer I started implementing Postfix and Exim solutions which are fine for low to mid volume email send outs, but you get in trouble when sending over 100k of mails per hour, let alone trying to send millions of mails in a day.

I will admit that back in the days I have worked on multi postfix installs on one server to create separate queues in order to separate mail streams, but even with that IO lead to be the main bottleneck of this solution.

Sending higher volumes

The next step up in the MTA business back then (2010-ish) were PowerMTA from Port25 and Momentum from Message Systems. Today both solutions are owned by Bird creating a nice vendor lock-in for mass mailing solutions.

As an upgrade to the Postfix solution we migrated to PowerMTA which is very capable of sending millions of mails. It can do rate limiting per domain (and even per queue) and it is very easy configurable. It also can respond to bounces in a sensible way and go in back-off mode if too much of the same bounces happen.

Several years later I also worked with Momentum which had a very advanced way to respond to bounces of ESPs. It was called adaptive delivery and a very nice feature which was miles ahead of PowerMTA to respond clever based on the actual bounce response.

For example if Gmail gives a bounce notification when sending a mail: “Our system has detected an unusual rate”. That means that you need to slow down your sending. Momentum was able to then slow down the sending speed or even suspend send out for 30 minutes or more.

And this is the part I love the most since we come at the technical part of email deliverability, which is making email delivery an easier job. You don’t not need to look at logs anymore in order to change the sending rate or move messages in a slower sending queue. With the adaptive delivery properly setup you can concentrate on the actual content of client emails for even better delivery.

Mass-mailing for all

Fast forward to 2022, when I started to implement a small scale email platform based on Postfix and I intended to use a custom made Postfix container configuration with dkim and rate-limit support. Even though that it could do most of the things (in a technical sense) that I required for improved delivery compared to a stock Postfix, those deliverability rules still required manual configuration.

With that in mind I went looking for other solutions and at a certain moment I found KumoMTA . After experimenting with it for some time to find out what KumoMTA was capable off, I joined the KumoMTA Discord in december 2023. KumoMTA replaced the Postfixes used in the small scale email platform to at least remove the manual work to update rate limits.

KumoMTA is a very promising mass-mailing solution for all ESPs, but it still is a relatively young project not having all the bells and whistles which PowerMTA and Momentum have. For me and my project it ticked all features I need and especially the shaping automation is something I was very happy about.

In discussions on Discord with Mike Hillyer about the shaping rules and how to make them, we both were on the same page that in 2024 the rules to react to bounce messages is actually common ground. Most big ESPs like Gmail, Outlook, Yahoo and for example Orange.fr do have postmaster pages explaining what each bounce notification stands for.

Mike’s way of explaining that the MTA is a commodity solution:
“The MTA is like a hotel’s plumbing: it better work, but no-one is picking their hotel based on the plumbing.”

With that concept in mind I started to build on the shaping automation helper in KumoMTA and worked on a shaping.toml file which tries to follow up on the most occurring bounce messages with appropriate measures. To make it the community shaping file in the KumoMTA package.

I started off with the big ones like Gmail, Yahoo and Outlook.com and then I added a few others which I came across while delivering mail. I am very proud to now say that in the latest dev builds you will be able to enable the community shaping.toml file to deliver emails better with KumoMTA.

If you are not running the latest versions of KumoMTA then you can still use the file by simply downloading the file from https://github.com/KumoCorp/kumomta/blob/main/assets/community/shaping.toml and use it in your own shaping.toml configuration. It should work in most versions of KumoMTA. I use it myself on the 2024.06.10.140609.74a0f2be -dev version.

Questions can be asked on the KumoMTA Discord and then specifically checkout ‘Solmea’.