A message broker is a middleware application that translates an email message from one proprietary format to another. This application helps aid in the smooth transition of messages between one messaging architecture and another.
For example, if one company uses Microsoft Exchange® as its email server software and Outlook® as the client, the message broker employed by Exchange® is used to communicate with external mail servers as the need to route messages arises. When email is sent out, Exchange® uses the standard Simple Message Transfer Protocol (SMTP) to send the message to the recipient server. By doing this, the message sent using Outlook® and Exchange® can be received by someone at an organization running Lotus Domino® and Lotus Notes®.
To the users of Outlook® or Lotus Notes®, the interaction is unnoticeable. This is all because of the message broker determining where the message is going and a standard protocol for sending the message to another server.
Message brokers are a part of the messaging solution. Microsoft Exchange® is an email server software that acts as a message broker by translating messages received via one protocol, HTTP, MAPI, or SMTP, to another protocol to be sent out. While most outbound email communication at some point uses SMTP to send the email, in the case of a messaging server like Exchange®, it depends on the client used with the system. If the client is Outlook®, the messages get to the Exchange® server using the Messaging Application Programming Interface (MAPI). Once on the server, Exchange must evaluate each message and determine if translation to another protocol is needed. If the recipient's mailbox is on the same server, there is no need to change the protocol and the message is delivered. If it is on another server and must traverse the Internet, the message is sent out via SMTP to the remote server, where it can be handled by a message broker to be delivered to the mailbox of the recipient.
A message broker is not the complete package that is a mail server or messaging server, but part of the application that allows the smooth mail flow from one individual to another. These middleware applications help keep the majority of the work involved in sending email messages on the server and out of the sight and minds of the people doing the sending. This process helps email stay seamless to its users and integral in day to day business operations.