Thank you for your reply even though I posted into the wrong category.
1) I prefer to use SQL Server.
2) I will use hMail's inline AV scanning.
3) I create email accounts to provide "From:" detail for out-going email messages my web-based application generates. The application really only needs an SMTP service. Users never logon and don't need POP3 as every email account forwards incoming messages to the user associated with the account.
My application regsiters volunteers, organizes committees and manages teams for schools. Each school has its own email account. Volunteers exchange email within their community and the "From:" portion of the header is "SchoolName"@TheSchoolVolunteer.org. Outgoing messages insert the originating volunteer's email address in the "Reply-to:" portion of the header.
So how to I estimate my RAM requirements?