class postfix-server {
  package { postfix: ensure => latest }
  service { postfix: ... }
  file { "/etc/postfix/master.cf": 
    source => "puppet:///postfix-server/master.cf",
    ...
  }
} 
class postfix-server::clamassassin-relay { 
  package { clamassassin: ensure => present }
  File["/etc/postfix/master.cf"] {
   source => "puppet:///postfix-server/clamassassin-relay/master.cf",
   ...
  }
}