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Edge always sits on the perimeter network which will receive all incoming and outgoing external email. Even though the Transport Services reside on the mailbox role in Exchange 2013 <b>external mail</b> does still flow through the CAS server. Below is a great link that illustrates the transport pipeline and how mail flows internally and externally. <br />
<br />
<a href="https://technet.microsoft.com/en-us/library/aa996349(v=exchg.150).aspx" rel="ugc">https://technet.microsoft.com/en-us/library/aa996349(v=exchg.150).aspx</a><br />
<br />
Will.
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Edge always sits on the perimeter network which will receive all incoming and outgoing external email. Even though the Transport Services reside on the mailbox role in Exchange 2013 external mail does still flow through the CAS server. Below is a great link that illustrates the transport pipeline and how mail flows internally and externally.

https://technet.microsoft.com/en-us/library/aa996349(v=exchg.150).aspx [https://technet.microsoft.com/en-us/library/aa996349(v=exchg.150).aspx]

Will.

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ok grt but if one user is on mailbox 1 and second on mail 2 then how mail flow will work between them?
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ok grt but if one user is on mailbox 1 and second on mail 2 then how mail flow will work between them?

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so any components like AD site and services etc used in this process.
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so any components like AD site and services etc used in this process.

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Exchange uses AD Sites and Service to tell Exchange what Site Path to use based on cost. This has nothing to do with the actual routing mechanisum built-in to Exchange, aside from telling Exchange which is the best path based on cost. <br />
<br />
Will.
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Exchange uses AD Sites and Service to tell Exchange what Site Path to use based on cost. This has nothing to do with the actual routing mechanisum built-in to Exchange, aside from telling Exchange which is the best path based on cost.

Will.

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You're Welcome.<br />
<br />
Will.
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let suppose i am having edge, cas and mailbox 2013 then what is the process of mail flow happens.
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let suppose i am having edge, cas and mailbox 2013 then what is the process of mail flow happens.

external and internal mail flow

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.