Kerio Mail Server (KMS, now Kerio Connect) is a good mail/groupware server with support for a staggering amount of clients and devices, including the ubiquitous Microsoft Outlook. Administration via the web/Mac/Win32 administration interface is a breeze and the server has modest hardware requirements.
Adminstration via the backend isn’t so easy though. If you want a report on who’s about to exhaust their quota - or already have - you’re out of luck. Making a list of all valid email addresses on the server is equally difficult, which means you have to do a lot of keyboard work to maintain user lists for e.g. an SMTP gateway or an antispam/antivirus appliance. KMS’ builtin LDAP does not return anything useful in this regard since it’s only meant to facilitate recipient autocompletion. I.e. you can’t ask Kerio’s LDAP for a list of email addresses.