Which is the Best Windows based email servers ( to be part of SAAS application)

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Hi,
We are creating a multi-tenant document Mgmt system used for document storage and other document related collaboration features along with mail in a SAAS model.
This application needs to support the following mail features
a. Dynamically creation/updation/deletion of Mailboxes for users
b. webmail related functionality like reply, forward, search mail, compose mail, delete mail, attach documents to mail, create/view/delete/update calendar events and create/view/delete/update task
c. Support for inbound and outbound rules.

Mailserver product should provide detailed API's, which would help in customing application

require your suggestion for finalizing the best windows mailserver to suit our requirement.

Thanks in advance for viewing this post and your response.

Thanks
Prasad