At Crossett Brook we are using iMail by Ipswitch.... having a web-based mail server is convenient for staff at home (we do not provide e-mail for students), and server-side storage is nice for those who use more than one machine.  iMail works as both an SMTP and POP server, so it can be accessed form Eudora, Outlook, etc. if users choose.  Our elementary school uses the standard Appleshare POP server, with Outlook.  Both of our servers run mail virus scans, so we have not had virus trouble (yet).  There is a $1K price tag for the newest version...