Selfhosted

Ive been using exim, hardly a change to my config in many years.so there is almost no point in having a webui to config. Works great. Front end with pfsense and pfblocker, add spamassassin, it is very, very, very solid once you get over the initial configuration hump. Migrate by copying the config file to the new server.

Heard good things about it, but if nothing changed, it still is pretty much a one-person project afaik. Thats what makes me cautious.

Full disclaimer : I've only ever used stalwart. Started with it and had no reason to change.

Been using it for about a year as a mail server for my services.

The set up is simple, configuration can be done through the webui or the conf files.
Creating users, mails, aliases etc is very straight forward.
Everything is pretty much painless, the documentation has almost everything you might need.
You can also configure automatic cert renewal. Something else I really like is that when you add a domain (you can have multiple) it also gives you the DNS configuration to add to your DNS server.

Honestly I'd say it's a solid choice. On my side it'll be part of futur projects where a mail server is needed.

I looked at Stalwart and was intrigued by it being implemented in Rust. I'm not sure if I backed away because at the time I'd have had to give up a webui for configuring it, because that would have felt like a step back in ease of use. But it sounds like there is one now.

I've used Mailcow-dockerized for about five years now and it's been super low maintenance and I think they test their updates very well because I've never had a problem. I've added Roundcube webmail to it, but their instructions for adding that are very easy to follow. It uses all the standard backend under the hood so I find that comforting as that's the stack I've used for 25 years anyway, I just wanted an easier way to maintain and update it and Mailcow fits the bill.

I've tested it (on NixOS). But just for two weeks. I'd say it's pretty impressive. Certainly works. It was just missing some important (to me) feature (forwarding mail to external mailboxes). But they've added it since, so I would like to try again. It doesn't seem to have all the bells and whistles (and I didn't have a look at the program code) but the basic features of a mailserver seem to be solid. I can't really comment on the sustainability of the project, quality of the documentation... I mean if your setup includes niche edge-cases, custom tweaking and hooking into other software, maybe stick with the popular choice. But if you just want a regular, more or less simple mailserver, I'd say go for it.

I can't comment on Stalwart but if you're looking for something reliable and easy to install on a VPS I recommend YUNOhost, which is a FOSS server OS that has email out of the box.