Service Broker is message queuing for SQL Server. It is used for sending emails and through Database Mails. You can read about SQL SERVER – Difference Between Database Mail and SQLMail here. Service Broker is feature which provides facility to SQL Server to send an asynchronous, transactional message.
So how it works is SQL Server uses Service Broker to send message from database to message queue. This message is then picked up by different processes to send this message to its destination. This separate processes can be using SMTP protocol. Sending and receiving processes can be done using entirely different system or SQL Server instance. It is not required that both the services are online at the same time. Sending services or Receiving services both are not required to be on line at the same time.
In summary, Services Broker follows traditional client-server architecture. Here client service initiates a conversation and receiving services takes messages and processes to send it to its destination.
Reference : Pinal Dave (http://blog.SQLAuthority.com)