Verily, Ikonia speaketh the truth (as usual
)
His listing of distros is a fair one for the application you have in mind, and given the relative complexity of compiling a latest patched copy of qmail, (which I overtlooked) sendmail and postfix are probably your best options. Sendmail is a standard, and ships with most distros, but some consider postfix to be a more flexible alternative.
If you are planning to install this in an enterprise environment, I would advise you to take a trip down to a large branch of Waterstones or a technical bookshop and have a browse through some of the many textbooks on the subject. Have a look on Amazon first to try and narrow your selection - there are many to choose from, both sendmail and postfix.
Installing the application is one thin g - getting it to run correctly - and securely is another - the last thing you want to do is leave an open relay running!
I have played with sendmail and got it to do what I wanted after a while (not actually running a mail server as such - for my simple needs it is too much hassle) but in an enterprise environment (that I assume you intend running it in) you need it to work correctly from the start!
What is it you actually want to do? Is it download several users' mail from your ISP's mailbox and then use the server to distribute it locally? If that is the case, you might find it easier just to buy more pop3 mailboxes from your ISP one for each user, although the server route is more flexible and gives you more control. If it is an IMAP facility you want to provide, then you will have to go the own server route.
(Sorry - you answered the last part while I was composing my post)