Preferred mail system/server for a company?

My advice: If you found a email system you don't hate, keep using it and consider yourself very lucky.

Without a lot more details as to your setup it's quite hard to make an informed comment but those are mostly over rated..

If your talking a handful of users in a generic small corporate office then the Windows Small Business stuff (Including Exchange/Outlook) can be reasonably compelling for a few reasons. Mostly it is something the users are familiar with and it is generic enough that support for issues is easy to find.

With respect to the FOSS solutions and others such as Zimbra; I'd steer clear of them unless you have a decent quantity of inhouse expertise in the technologies involved. Finding an outsourced company to support your generic Outlook setup will be trivial, finding the same for your Zimbra/Scalix/Custom IMAP solution hack won't be.

As to Gordano; I've never heard of them and yet their website claims to be a leading collaborative software solution. That's the kind of statement that gives me pause.

For reference professionally I've worked with (as an administrator) GroupWise, Exchange, Zarafa, Zimbra and a raft of mta's (exim, postfix, et al) and storage backends (cyrus, dovcot, washington) and for all of them Exchange gives the most bang for buck with the least effort and easiest generic support path.

Alternatively you could entirely outsource your problems and go with something like Google Apps, hosted Zimbra, or find a local outsourced admin team and have them install whatever they are happy to support.

Kyle has it right - If you have a mail solution you don't hate and it has no glaring shortcomings you have found your system, don't change if you don't have to.

Out of your list I would discount Qmail: As far as I'm aware djb doesn't maintain it anymore, and while it's got a great community and lots of "modernizing" fixes (vpopmail, etc.) it does seem to be a niche market and I find that its quirks tend to outweigh its benefits.

Also if you're not married to the "djb way of doing things" (daemontools and the like) qmail will be pretty heavy culture shock.

For what it's worth: At work we're running qmail, soon to be replaced by an XServe running Apple Mail Services (essentially a fancied-up version of postfix, but we're switching for other reasons). At home I run sendmail (because a long time ago I memorized the Bat Book, and its suck level hasn't exceeded my threshold for replacing it yet).