When you install a Mailbox server running the Transport service, two Receive connectors are created. No additional Receive connectors are needed for typical operation, and in most cases the default Receive connectors don't require a configuration change. These connectors are the following:

• Default Accepts connections from Mailbox servers running the Transport service and from Edge servers.
  
  
• Client Proxy Accepts connections from front-end servers. Typically, messages are sent to a front-end server over SMTP.
  
  

Each connector is assigned a TransportRole value. You can use it to determine the role the connector is running in. This can be helpful in cases where you are running multiple roles on a single server. In the case of each Receive connector previously mentioned, their TransportRole value is HubTransport.

To view the default Receive connectors and their parameter values, you can use the Get-ReceiveConnector cmdlet.

During installation, three Receive connectors are created on the Front End transport, or Client Access server. The default Front End Receive connector is configured to accept SMTP communications from all IP address ranges. Additionally, there is a Receive connector that can act as an outbound proxy for messages sent to the front-end server from Mailbox servers. Finally, there is a secure Receive connector configured to accept messages encrypted with Transport Layer Security (TLS). These connectors are the following:

• Default FrontEnd Accepts connections from SMTP senders over port 25. This is the common messaging entry point into your organization.
  
  
• Outbound Proxy Frontend Accepts messages from a Send Connector on a back-end server, with front-end proxy enabled.
  
  
• Client Frontend Accepts secure connections, with Transport Layer Security (TLS) applied.
  
  

In a typical installation, no additional Receive connectors are required.

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Mailbox Server running with Transport Service
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Default : 用来给本身或者其他MBX或者Edge服务器上的transport service提交邮件 。
Client Proxy :用来给Front-End服务器的transport service提交邮件。


FrontEnd Transport Server
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Client Frontend (TCP 587):SMTP客户端通过 TLS的加密通道提交邮件。
Default Frontend (TCP 25):SMTP客户端通过 TCP端口25提交邮件。缺省情况下，邮件从外部服务器传输就是通过这个端口的。
Outbound Proxy Frontend :如果在后端服务器上有开启 FE proxy功能, 那么这个receive connector就是用来给后端服务器的send connector 来提交邮件的。