LinuxQuestions.org - Mail server installation

I am currently installing a new mailserver for around 500 users and I was hoping that I could make use of some of the experience available. I'm an advanced Linux user myself, so this is not a question of how to install, but one of *what* to install so that it will fit my needs.

I want to provide users with the following:
- real user account for potential shell access and to allow easy disk quotas
- mail access through IMAP, POP3 and shell access for those allowed
- incoming mail should be stored in a mail spool directory so that exceeding quotas will immediately bounce e-mail
- IMAP e-mail folders should be stored in the home directory. That would include moving the mail from the spool to an IMAP inbox before it can be read.
- I want IMAP folder to be stored in a fixed location (e.g. $HOME/.imapfolders ) and have the IMAP server configured in such a way that it won't show anything else (like every single other file in the home directory as most do by default).
- I probably want to use POP3 as a gateway to the IMAP inbox so that it can't access the mail spool file before it's in the users own space.

I'm curious to find out what combinations of MTA, local delivery agent and POP3/IMAP deamon will fit these requirements. Especially the fixed IMAP folder location.

Mail storage method (mbox/MH/maildir) is not all that important to me.

Also, I want to provide webmail. But SquirrelMail should be fine through IMAP in any case.

Thanks in advance for your suggestions.

Edwin