I am running unix CentOS 5 and looking for free mail server that has GUI web interface for managing accounts. It be used with windows based clients on outlook express.
The Zimbra communication suite is apparently pretty popular - I've been investigating it for use at my company, and it pretty much meets all the standard Outlook/Exchange work-alike requirements.
Maybe have a look at Proxmox Mail Gateway: http://www.proxmox.com/products/proxmox-mail-gateway
They have a Free license which appears to restrict to one domain.
Lots of people, including myself, use their free Proxmox VE (Virtualization Environment) product which provides a similar looking GUI web interface. If the quality of that product is any indication, it is probably worth your while to check out the Mail Gateway.
Their datasheet includes a table that shows what the Free version does not include.