E-Mail Archiving Tutorial
Email Archiving enables you to store every email message to an archiving storage area. You can define granular policies and requirements that would determine what type of emails you would want to store away, for how long these messages should be stored before being automatically purged and other criteria can be assigned.
Some reasons why you would store your emails would be for regulatory compliance, long term storage (taking the burden away from your email server), email archivers have powerful search facility allowing users to find messages based on a number of options.
Email archiving products usually come in stand alone solutions as a software, appliance or as a service in the cloud. However some vendors have integrated these solutions within their email security solutions as well providing email security and archiving all from one solution. There are a number of ways archiving software integrates with your email server. On basic method is you can configure your email server so that when it processes a message it also sends a copy of it to the archiving solution for long term storage.
Further Reading
A list of Email Archiving Vendors