Have not looked at those others.  I have worked with SmarterMail and Exchange, and am beginning to look at postfix (but only as a possible email filter).

The interesting email client protocols are MAPI and EAS, and MAPI is the only client protocol that works acceptably with very large mailboxes.   SmarterMail says they are the only non-Exchange product to support MAPI, and I have no evidence to the contrary.

Compared to Exchange, SmarterMail is cheaper and simpler.    Like Exchange, it runs on Windows, which is important if you lack staff with a Linux skill set  Exchange worked great for organizations with a PhD in Microsoft, until Microsoft forced all of them to give away their mail system to Office365.

So, SmarterMail can be a very good choice.

However, setting up your own mail system has little to do with mail and everything to do with spam filtering.   Doing that well is hard and time-consuming.  I have a personal grudge match with a bunch of vendors who offer spam filtering but do not offer the capabilities necessary to do it properly, so I don't even know what to recommend for someone in your position.  I use two commercial spam filters because they do content filtering pretty well at a tolerable price.  For sender filtering, I am a fan of using SmarterMail with Declude as an incoming gateway, but customizing Declude to my satisfaction has taken a lot of time and effort.    Are you sure that you want the headache of managing your client's email?   It is not a simple question.

IMHO,  SmarterMail is pretty good at the Enterprise level.  Basically, if you want ActiveSync/MAPI and a real MS Exchange option, I would recommend it.  For smaller builds, I don't think it's worth the hassle of running Windows, but that's a personal preference of mine.  Zimbra is good, but gets out of control at times, which is why I think they tend to lean towards hosted options now.  If you've got that handled, than I would also check out Open-Xchange.  My main grief is that all of these options rely on plugins for Outlook, which I really don't like.

So for me, I would stick with just rspamd, exim, and dovecot with maybe a horde frontend on little builds, but for full functionality on the cheaper side SmarterMail is definitely easier than MS Exchange.

Thanks for the kind words, guys. Makes us happy to see someone who is a Linux person preferring SmarterMail on Windows :) for larger installs.

As an aside, we're transitioning SmarterMail to more modern and cross platform .NET 6, which means a Linux version of SmarterMail is getting closer and closer. 

So, is the Linux version available now or soon to be? 

I think the following three features would be very useful to have. I've been waiting for them since 2017. I'm hoping you'll put them on the board: 1. scheduled delivery. This is a great feature. It's not enough that Outlook offers it. 2. Being able to inset the user's picture to the signature in addition to company's logo. 3. The ability to send the email as a postcard. I do use this feature often for various reasons: thank you, birthday, holiday, etc. Of these three, the first one is of the most immediate interest. Could and will you develop it?

The issue with Client Disconnect is relevant to this question.  (1) SmarterMail provides frequent releases.   Sometimes that is a frustration, but often it means that a problem is patched more quickly with SmarterMail than with any comparable product.   (2) SmarterMail adapts pretty well to the external world.   They figured out that Microsoft had a bug with client disconnect, so they provided a workaround to the bug (which is working nicely, thank you!).  I think the only complaint is that their release notes tend to be too abbreviated, so we did not understand why the "deliver on disconnect" option was important.