Hello everyone,

So many times, people ask me about connectors in Exchange 2013 / 2016. Even thought topic is not so complicated, it is a very important to know how Exchange connectors works, especially receive connectors. I will try to explain you that in short lines.

Exchange 2016 in multi-role server. Unlike previous versions, all the roles are on a single server now. Of course, all receive connectors are on single server now. Let’s start 🙂


Frontend Transport

This service acts as a stateless proxy for all inbound and (optionally) outbound external SMTP traffic for the Exchange. The Front End Transport service doesn’t inspect message content, doesn’t communicate with the Mailbox Transport service, and doesn’t queue any messages locally.

  • Client Frontend – Connector accepts secure connections on port 587, with TLS applied. This receive connector is used by IMAP and POP clients. Connector proxies connection from IMAP and POP applications to HubTransport receive connector called Client Proxy.
  • Default Frontend – Emails sent from Internet are received by this transport service on port 25. Received emails will be send to HubTransport receive connector called Default connector on port 2525.
  • Outbound Proxy Frontend – If you have checked proxy through client access server option in Send connector, all outgoing emails will be received by this connector, and send to destination. It receives emails on port 717.


Hub Transport

The Transport service handles all SMTP mail flow for the organization, performs message categorization, and performs message content inspection. The Transport service routes messages among the Mailbox Transport service, the Transport service, the Front End Transport service, and (depending on your configuration) the Transport service on Edge Transport servers.

  • Client Proxy – Hub transport service which accepts emails sent from frontend services and sends to mailbox transport service on port 465. This receive connector accepts proxied POP and IMAP connections sent from front end transport from receive connector called Client Frontend.
  • Default – This connector accepts incoming emails from front end transport service and sends to mailbox transport service. Similarly, accepts outgoing emails from mailbox service and uses send connector to send email or sends to receive connector called Outbound Proxy Frontend if proxy through client access server option is enabled as talked before. It accepts emails on port 2525.



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