We have done hosted solution on centos 6 64Bit:

Mysql
Openldap(companies, groups, users schema and PHP based tool for management)
Nginx(with PHP)
Zarafa 7.0.7
Postfix(dspam, amavisd, headers filter, smtp user auth by LDAP)

System can serve up to 4000 users by 2GB stores.

2x Xeon
64 GB ram
8TB raid 10