A messaging server handles messages between applications using a messaging API. Microsoft Exchange is a common example. Messages are stored in a queue and routed to the appropriate recipient. If there are connectivity issues, a delivery delay message may be received. Other messaging servers, like Qmail, require less administration. When a message is sent, it enters the message queue and is processed to determine the recipient’s mailbox location. If the recipient is local, the message is delivered immediately, otherwise DNS is contacted to locate the server on the Internet.
A messaging server is an application that handles messages between two or more applications. These messages are passed to the middleware application using a messaging API (MAPI). Messaging servers are able to store messages in a queue until they can be delivered to recipient applications.
A typical example of a messaging server application in many organizations is Microsoft Exchange. Exchange sits on a server and waits for messages to be routed to its queue. From there, the middleware determines one message at a time whether the message should be routed locally or sent to another messaging server for delivery. If the message belongs to an on-premises recipient, the message is delivered to the on-premises mailbox almost immediately. If the message belongs to a remote message store, the Exchange server will query other application servers in an environment, such as DNS, to locate which server the message belongs to.
If there are connectivity problems between two messaging servers or the sending server cannot determine the location of the remote server, the person sending the message may receive a delivery delay message from their server. Usually this message will let that person know that the server is having trouble, but to keep trying for a specified number of retries or days.
There are other types of messaging servers or email servers. Some, like Qmail, run on the Linux operating system and require much less overhead and administration than an Exchange server. This makes them ideal for ISPs that host large numbers of email accounts for customers. Many companies use Microsoft Exchange for messaging because it is very reliable and works well with other Microsoft products used by companies.
When an individual sends a message, Outlook (or its messaging client) passes the message to the messaging server where the message enters the message queue to wait its turn to be processed. If there is a lot of mail in the queue, this process may take some time, although usually nobody will notice the transition. When the message is processed, the messaging server determines the location of the recipient’s mailbox. If the mailbox is local to that server, the message is delivered immediately to its destination. If the recipient is not local to the server, DNS is contacted to help locate the server on the Internet. When the server is found, the sent email message is transferred to the receiving server where the process starts locally from a queue to handle the message. Once the correct mailbox has been located, the message is delivered.
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