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.