Qmail or Zmailer

[Eden Akhavi <eden.akhavi@fci.co.uk>]

We are looking to build a high performance mail-cluster for pop3, imap4 and
smtp. The system uses an LDAP database to validate and deliver the users mail.

The system needs to scale to around 500,000 users - the questions I have:

1. Which is better for this Qmail or ZMailer
2. What would be the best way of creating a high availability service -
multiple POP3, multiple SMTP, etc pointing to common NFS mount?
3. Has anyone found any major issues with Zmailer which stop it scaling.

Thanks

Eden