I'm building a linux server that has to be email server for a domain. It has to do SMTP, POP and/or IMAP.I'm looking for some web based tools to administer these services. And we are planning to give web based email access to clients (squirrel mail???), so is there a free tool for Linux that will let us give web based email access to clients?

All I want is programs for SMTP, POP, IMAP and for managing these services ?? I know a few but just want to get opinion and choice of folks here. And are there any anti-spam and anti-virus s/w that I can install on this server (free or commercial) -- need good programs?? Does sendmail come with one?? 

And is there a tool (something like a control panel) that when we give access to our clients, they can add users (email users) to the domains that we are hosting? Please suggest me appropriate tools (free or commercial)

Thanks