Looking for a mail server other than Microsoft Exchange:
Requirements are as following:
1. Mail server which can hold 5000-10000 users with ease.
2. Good looking web mail interface with Calendar, Scheduler, Messenger and etc.
3. Fail over setup to meet high availability.
4. Easy migration, Export and Import of mails.
5. IMAP and POP supported.
6. Effective Spam Controller.
7. Good support.
Please suggest some top mail servers