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Every HUMAN BEING that accesses a mailbox on the exchange server needs a mailbox. &nbsp;Not user account. &nbsp;Not mailbox.<br />
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That said:<br />
Licensing Disclaimer<br />
License information provided here is &quot;best efforts&quot;. &nbsp;The comments of the respondents are based on interpretation of the license agreements and their knowledge of the particular laws and regulations in their geographic location. &nbsp;Laws in your location may invalidate certain aspects of the license and/or licenses can change. &nbsp;&quot;They told me on Experts-Exchange&quot; will not be a valid excuse in an audit. &nbsp;You need to contact the license granting authority to confirm any advice offered here.
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Every HUMAN BEING that accesses a mailbox on the exchange server needs a mailbox.  Not user account.  Not mailbox.

That said:
Licensing Disclaimer
License information provided here is "best efforts".  The comments of the respondents are based on interpretation of the license agreements and their knowledge of the particular laws and regulations in their geographic location.  Laws in your location may invalidate certain aspects of the license and/or licenses can change.  "They told me on Experts-Exchange" will not be a valid excuse in an audit.  You need to contact the license granting authority to confirm any advice offered here.

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I want to know How many CAL i need &nbsp;purchase if I will have a Exchange 2013 with 400 Mails boxes?
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I want to know How many CAL i need  purchase if I will have a Exchange 2013 with 400 Mails boxes?

Exchange 2013 License

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Windows Server 2008 and Windows Server 2008 R2, based on the Microsoft Vista codebase, is the last 32-bit server operating system released by Microsoft. It has a number of versions, including including Foundation, Standard, Enterprise, Datacenter, Web, HPC Server, Itanium and Storage; new features included server core installation and Hyper-V.

Windows Server 2008

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.