However, unlike Postfix, you can actually configure OpenSMTPD yourself without breaking it. Postfix suffers heavily from a Byzantium configuration that everyone is copy-pasting because understanding how to create a configuration from scratch requires man-months of reading their docs.
The biggest downside to OpenSMTPD is the portable version is weakly supported, so it's only useful if you are running it on OpenBSD, which is a shame, really, because it's probably what most people would prefer if they could run it on their preferred OS.
At the other end of the spectrum, Postfix is incredible - it's FOSS, and it does all that.
Another nice feature is what I would call its architecture: it does not try to be a service manager (instead, you are supposed to spawn it from your own favorite service manager, be that systemd, docker, or anything else, and you can hence really lock it down), hence it does not require root to run and does not rely on using system accounts by default.
I've only used it for very basic setups, but for those I can highly recommend it.
I haven't done a detailed comparison, but going how the OpenBSD guys go with other re-implementations (e.g. LibreSSL vs OpenSSL), I'm going to guess that it will have a reduced feature-set and capabilities.
I've nothing against the OpenBSD guys, but sometimes its a case of "right tool for the job". OpenBSD is great for firewalls and routers. Of course OpenSSH is without competitor, its just awesome.
But (some of) the rest of the OpenBSD toolset I'm not too sure ... for example Chrony is lightyears ahead of OpenNTPD etc.
Postfix is an excellent MTA. Written from a security-first mentality. Its widely deployed, and hence battle tested. As a result of its wide deployment its pretty much a case of "everyone knows how it works", the Postfix mailing list is excellent in terms of getting support.
TL;DR personally I see no reason to replace Postfix with anything, be it OpenSMTPD or anything else.
"OpenSMTPD is a fairly complete SMTP implementation."
Technically OpenNTPD is a "fairly complete implementation" too, but its a waste of time compared to Chrony.
It's also explicitly intended not to solve every NTP use case but rather just the common "I want my laptop's clock to be roughly accurate" use case.
[1]: https://poolp.org/posts/2021-01-31/january-2021-opensmtpd-li...