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Hi All, thanks for your replies. @John: What does RDS stand for?
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Hi All, thanks for your replies. @John: What does RDS stand for?

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Thank you very much for your times and answers!
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Thank you very much for your times and answers!

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Dear Wizards, regarding this post: <a href="https://technet.microsoft.com/en-us/library/bb232170(v=exchg.160).aspx" rel="ugc">https://technet.microsoft.com/en-us/library/bb232170(v=exchg.160).aspx</a>, we have to purchase license for each mail user, am I right? If the number of user is increased in the future, what happens? <br />
<br />
For example: Now we have only 20 users, we will purchase license for 20 users, right? But next month there are more 5 ppl, so can we still use Exchange server (Outlook, OWA) for these 5 new users? Many thanks in advance.
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Dear Wizards, regarding this post: https://technet.microsoft.com/en-us/library/bb232170(v=exchg.160).aspx, we have to purchase license for each mail user, am I right? If the number of user is increased in the future, what happens? 

For example: Now we have only 20 users, we will purchase license for 20 users, right? But next month there are more 5 ppl, so can we still use Exchange server (Outlook, OWA) for these 5 new users? Many thanks in advance.

MS Exchange 2016 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

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.

Outlook

license

An email client, email reader or more formally mail user agent (MUA) is a computer program in the category of groupware environments used to access and manage a user's email. A web application that provides message management, composition, and reception functions may internally act as an email client; as a whole, it is commonly referred to as webmail. Likewise, email client may be referred to a piece of computer hardware or software whose primary or most visible role is to work as an email client. Email clients can also have other systems, like calendars, notes and contact managers.

Email Clients

Within Internet message handling services (MHS), a message transfer agent or mail transfer agent (MTA) or mail relay is software that transfers electronic mail messages from one computer to another using a client–server application architecture. A MTA implements both the client (sending) and server (receiving) portions of the Simple Mail Transfer Protocol (SMTP). The terms mail server, mail exchanger, and MX host may also refer to a computer performing the MTA function. The Domain Name System (DNS) associates a mail server to a domain with mail exchanger (MX) resource records containing the domain name of a host providing MTA services.