HELP: Postfix server configuration - Verify users in Active Directory, POP3/IMAP access for Outlook, and configure Webmail

Hello,

I work for a company that has limited resources and few employees.  We have a Windows 2003 domain and want to incorporate a mail server into our network.  I've worked with Postfix in the past, however, only as a mail relay to an Exchange server.  Now I want to configure a Postfix server that will store all mail for the domain and where the users can access their messages via Outlook and/or webmail.

I have configured Postfix with OpenLDAP, and I'm able to verify users in Active Directory from my FreeBSD system, however, none of the verified users have mailboxes in FreeBSD.

My questions are:

(1)  Which POP3/IMAP is the most secure, yet easiest to configure/maintain?
(2)  By installing a POP3/IMAP server, will that autocreate the mailboxes based on validation through LDAP, or do I need to configure that in the server or create a script?
(3)  So far I am using simple authentication to access AD from FreeBSD.  In order to secure the LDAP queries I'm assuming I need to enable SSL/TSL support on my domain, create a certificate for the domain, enable SASL support on my FreeBSD server, then test the authentication with ldapsearch?  If so, does anyone have a HOWTO already created?
(4)  What is the best Webmail server available?
(5)  Once all this is completed, would there be any ramifications to installing SpamAssassin afterwards?

Hopefully someone out there has worked on this kind of configuration.  Any assistance will be greatly appreciated.

Thanks,

Dave