Actually, you don't get the 5000 user version for $89 you get the 5 user
version. The "unlimited/5000" version is several hundred more.
I use VOPMail: www.vircom.com/vopmail
No user count.
Works off your existing Radius database (or ODBC)
developing a web interface to read/write mail and adminstrate for domains;
Professional, friendly company that focuses on serving ISPs.