[ubuntu] How to chose mail server?

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http://ubuntuforums.org – Dear all I am planning to run an Ubuntu server as a mail server for our office. Our goal is to have an email server that connect to our web mail server and pull all the emails from server. Our employee don't have access to internet but we need all of them use email. So we want to have a local mail server. Everybody will connect to local mail server and our local mail server will connect to our web mail server. Now my question is that which mail server do you suggest to use? Thank you so much (Hardware)