What kind of E-mail server should I use?

Hi there. I'm working at a small firm (20 people) and we're using Mdaemon (www.altn.com) mail server for Windows. It is installed on a machine with Windows 2003 Server OS. However it is a bit old and we are thinking of an upgrade. We are not satisfied with its work, because we have to install additional software that helps MS Outlook to use IMAP with the server (it works fine with POP3, but we need IMAP) and the software is a bit buggy. (E-mails dissappear; some doesn't show up in Outlook, but are on the server; if you copy/move e-mails from folder to another it hangs and you have to restart outlook...). So we are looking for a new Windows based E-mail server that uses IMAP and works fine with MS Outlook. What do you think I should look for? The Partners in the firm receive about 50 e-mails per day and about 50 spam e-mails too. So it will be nice if it has decent spam defence.