KumoMTA

Pros: KumoMTA has easily the best documentation that I have ever read. It is super customizable because of the Lua scripting and if you ever get stuck, there's a great community that will help you out.

Features like webhooks are a blessing and once everything works, it works amazingly well.

Cons: It's a lot to take in, knowledge about email is mandatory. Examples in docs could be improved.

Overall: When I first started using KumoMTA, it took some time to get used to it. However, after all these months I can say I am beyond happy we opted for KumoMTA.

Pros: High performance, extremely stable, and open-source, giving enterprises more options.I replaced Momentum with KumoMTA, which saved me a significant amount of costs. Moreover, Momentum’s support has been getting worse.

Cons: Configuration items lack real, effective examples and guidance on how to better implement certain features, methods, and so on.
Out-of-the-box configurations are limited; this part could actually be developed into a marketplace to enable community participation in co-creation.

Overall: After a period of use, although it was difficult at the beginning, once you become familiar with it, you’ll find that KumoMTA is truly stable and reliable. I hope it keeps getting better.

Pros: There's a lot to like with Kumo: easy to spin up instances, easy to scale instances, lightweight, great webhook/event support, and their community is kind so any questions I have get answered quickly in their Discord.

Cons: There's no UI/frontend so it's very much for devs. This is fine because we built our own MTA monitoring software.

Overall: We're rolling this out across our ESP/fleet at the moment. It's been a joy to use and we recommend it to all our friends in the industry.

Pros: I love how quick and easy it is to setup KumoMTA.

It gives you total control over every part of the message delivery which helps you to transform and even fix messages that don’t follow RFC standards.

The configuration system is easy to use. It's extremely flexible, thanks to its Lua scripting module.

Well documented, scales beautiful. The webhook (logging) system is also great which makes collecting analytics and statistics really simple.

I really appreciate the level of personal support that I receive and I am truly grateful for it. In the past, I had a small feature request which was quickly incorporated into the software in under 24 hours.

Cons: There's a bit of learning curve at the start, especially for those new to Lua, but this is true for every software/MTA.

Apart from that, my experience with KumoMTA has been smooth sailing.

Overall: A beautiful software for anyone who wants to run production-grade email infrastructure. Stable, fast and modern, and an amazing community.