Over the last several months there has been significant chatter around what is background database maintenance and why is it important for Exchange 2010 databases. Hopefully this article will answe...
Updated Jul 01, 2019
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@ExchangeRox - the items Ross listed that occur 7x24 really are all about the database itself, specifically regarding defragmentation, page zeroing, etc... There are other tasks performed by Exchange for several versions now during a "maintenance window"
such as Item Tombstoning, deleted mailbox cleanup, deleted item retention cleanup, etc...
While I haven't found one for Exchange 2007, this list for Exchange 2007 should give you a clearer view of all of the maintenance tasks Exchange 2010 performs during the scheduled window above and beyond basic database maintenance:
http://technet.microsoft.com/en-us/library/bb123760(EXCHG.80).aspx
technet.microsoft.com/.../bb123760(EXCHG.80).aspx