There are a number of reasons why email should be archived, including legal, business and practical reasons. An important part of archiving is search and restore of valuable information. Fortunately a number of solutions are available some of which are highlighted below:

Email Archiving Solutions

Athena Archiver
Link: http://www.athenaarchiver.com/

Barracuda Message Archive
Link: http://www.barracudanetworks.com/ns/products/archiver-overview.php

C2C Archive One
Link: http://www.c2c.com/site/products/archiveone/default.asp

Clearswift MIMEsweeper Archiving
Link: http://www.clearswift.com/products/msw/archiving/default.aspx

CommVault Archive
Link: http://www.commvault.com/products/archive.asp

Cryoserver
Link: http://www.cryoserver.com/

EMC Email Xtender
Link: http://www.emc.com/products/family/email-xtender-family.htm

Fortiva Archiving
Link: http://www.fortiva.com/

GFI MailArchiver
Link: http://www.gfi.com/mailarchiver/

HP Email Archiving
Link: http://h18006.www1.hp.com/products/software/im/ediscovery_compliance/rim/index.html

IBM Common Store
Link: http://www.ibm.com/software/data/commonstore/

MessageLabs Email Archiving Service
Link: http://www.messagelabs.co.uk/products/archiving/

MessageSolution Enterprise Email Archive Suite
Link: http://www.messagesolution.com/email_archiving.htm

Mimosa Nearpoint
Link: http://www.mimosasystems.com/html/prod_nearpoint.htm

Quest Software Archive Manager
Link: http://www.quest.com/archive-manager/

Symantec Enterprise Vault
Link: http://www.symantec.com/business/products/overview.jsp?pcid=pcat_storage&pvid=322_1

Zantaz EAS Enterprise Archiving System
Link: http://www.zantaz.com/products/archiving/enterprise-archive-solution/index.htm

If you are aware of any other email archiving solutions please let me know and I will add them to the list.