For Client we have searched the internet and found and tested a nice feature packed email server for our clients.
The product is hMailServer and can be downloaded for free from http://www.hmailserver.com/
We also recomend if you are using a dedicated machine as your mail gateway that you also use ClamAV for virus detection in emails.
ClamAV can be downloaded from http://www.clamwin.com/
Both products require intermediate knowledge in internet protocols and configuring by a professional is advisable.
We offer a configuration and installation service: contact email@example.com for more information.