hello,

I will buy 20 dedicated server.(Private Rack)

I want to use;
Server 1 for Mail Server (MerakMail Server or Sendmail)
Server 2 for MySQL Server
Server 3 for MS SQL 2000 Server
Server 4 for DNS Server (BIND)
Server 5 and others for Plesk 6.5 & 7 Control panel and Web,FTP Servers.

How can I configure plesk for secondary mail, mysql, mssql and dns server?

I will use RAID 1 for backups. All of them Dual Xeon servers.

You can not or it will be very difficult because no control panel has features for this. The redundance problem is difficult and it must be solved based on case to case analysis.