Microsoft SMTP Service uses the SMTP protocol to transport and deliver messages. Messages transferred between remote mail servers and the Drop directory designated for the default domain. Client and server connections to SMTP do not involve user interactions and tend to be fairly short. Microsoft SMTP Service is designed to provide quick turnaround on connections to maximize messages-per-second processing.

How Microsoft SMTP Service Works?
When a message is placed in the Pickup directory or comes in through the designated TCP port, it is placed in the Queue directory. If the recipients are local, the message is delivered. If they are non-local, the message is processed for remote delivery.

Local Message Delivery

If a message is determined to be for a local recipient, the message is moved from the Queue directory to the Drop directory designated for the default domain. Once deposited in the Drop directory, the Microsoft SMTP Service delivery cycle is complete. The Drop directory can be designated on the Domain Properties property sheet.

Remote Message Delivery

The following list describes the processes that Microsoft SMTP Service uses to deliver remote messages:

  • Sort messages and queue for delivery : Messages remain in the Queue directory. They are sorted by domain so Microsoft SMTP Service can send them as a group. This enables Microsoft SMTP Service to optimize connections by delivering multiple messages in one session.
  • Determine if the receiving server is ready to receive messages. Microsoft SMTP Service attempts to connect to the receiving mail server. If the server is not ready, the message is returned to the queue, and delivery is attempted again at designated intervals to a maximum number of attempts. The settings for maximum retries and retry interval can be set on the Delivery property sheet.
  • Verify recipients : Each message recipient is verified. If a recipient is not verified, a non-delivery report (NDR) is generated for that recipient. You can designate where to store undeliverable messages and NDRs on the Messages property sheet.
  • Send the message : The message is sent. Once the receiving server acknowledges the transmission, Microsoft SMTP Service delivery is complete. Delivery options are described below:
  • If you have enabled Transport Layer Security (TLS) encryption for remote delivery, Microsoft SMTP Service encrypts all outgoing messages. You can also enable or disable this function for a specific remote domain. TLS encryption can be set on the Delivery property sheet by clicking the Outbound Security button and clearing or selecting TLS encryption.
  • All messages can be sent to a smart host, which then sends them to recipients. The smart host can be designated on the Delivery property sheet.
  • Messages destined for a remote domain can also be delivered using a specific route. This option enables you to specify a route that may be quicker and less costly than a direct link. This option overrides the smart host setting. Route domains can be set on the Domain Properties property sheet for the remote domain.