The Front End Transport service has a default Receive connector named Default Frontend that's configured to listen for inbound SMTP connections from any source on TCP port 25. In Exchange Server, the Front End Transport service and the Transport service are always located together on Mailbox servers. You need to create the dedicated Receive connector in the Front End Transport service, not in the Transport service. Restricting access to the Receive connector is critical, because you don't want to configure the server as an open relay. Don't attempt to add anonymous relay capability to the default Receive connectors that are created by Exchange. You need to create a dedicated Receive connector to specify the network hosts that are allowed to anonymously relay messages, so you can exclude anyone or anything else from using the connector. Here are some key considerations for the anonymous relay Receive connector: In Exchange Server, you can create a dedicated Receive connector in the Front End Transport service on a Mailbox server that allows anonymous relay from a specific list of internal network hosts. On the other hand, anonymous relay is a common requirement for many businesses that have internal web servers, database servers, monitoring applications, or other network devices that generate email messages, but are incapable of actually sending those messages. Open relay servers are eagerly sought out and used by spammers, so you never want your messaging servers to be configured for open relay. This behavior masks the original source of the messages, and makes it look like the mail originated from the open relay server.

Messaging servers that are accidentally or intentionally configured as open relays allow mail from any source to be transparently re-routed through the open relay server. Open relay is a very bad thing for messaging servers on the Internet.

550 5.7 1 unable to relay exchange 2010