Four Ways

Messages flow into hub transport server in four ways.

Way one:
Receive Connector via port 25 (SMTP) or receive connector via port 587:POP3 and IMAP4
Connectors handle messages flowing in from other hub transport servers.

Routing Group connectors allows messages to flow in from legacy Exchange Servers in a mixed organization.

Way two:
When MAPI clients connect to mailbox servers and send messages, Store Driver fetches the messages from the Outbox folders in user mailboxes.

Way three:
Pickup or Replay directories

You can create a well-formed SMTP messages and place them in \transportRoles\Pickup directory. Every 5 seconds, Exchange checks the pickup directory and attempts to process any file with an .eml extension.

The Replay directory is used to resubmit exported Exchange messages and to receive messages from foreign gateway servers.

example:message.eml

To:Kaiming.Liao@gmail.com
From:Alan@1ask2.com
Date:5 May 2011 05:45AM
Subject:Four way delivery
MIME-Version:1.0
Content-Type:text/html;charset="iso-8859-1"
Content-Transfer-Encoding:7bit

<HTML><body>
<h3>
Message is from the Pickup directory.
</h3>
<table>
<tr><td>Day</td><td>month</td></tr>
<tr><td>23></td><td>May</td></tr>
</body></html>

1

Save the message.eml into C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Pickup.

Set-TransportServer -identity Ex2 -PickupDirectoryPath "C:\Program Files\Microsoft\Exchange Server\V14\TransportRoles\Pickup"

Way four:
System-Generated (transport agents-generated) messages:

Nondelivery message, delivery receipts,transport rules, Journal reports, etc.