pop server in an ISP environment

[Muljawan Hendrianto <muljawan.hendrianto () siemens com sg>]

Hello all,

I would like to have your opinions regarding pop server set-up in an ISP environment, what would be the common software 
used, authentication type etc.

I am thinking about using QPOPPER+procmail, but some people say that it is not scalable because its authentication is 
based on /etc/passwd. And in Unix environment there is certain recommendation not to have more than 5000 users in 
/etc/passwd file.

Another issue is high availability, does any body use server clustering in an ISP environment?  
I am thinking about having a Sun Cluster for these pop servers, but will I need a special HA agent ?
AFAIK, the qpopper daemon will be initiated by the users (via inetd) when they are accessing their mailbox.
Hence a Sun cluster will not need a special agent because, if one of the servers crashed, the clustering software 
should 
manage to redirect any traffic to the second server, so that the users will connect transparently to that qpopper 
daemon in the second server.
Does clustering in the real world can do the above scenario?

What about LDAP for user's authentication ? Is it recommended to use ?

Any suggestions/hints will be greatly appreciated.

ps: the system should be able to accomodate about 20000 users.

regards,
Muljawan