Build numbers and release dates for Exchange Server

Microsoft Exchange Server 2003                     6.5.6944

October 2003

Microsoft Exchange Server 2003 SP1              6.5.7226

May 2004

Microsoft Exchange Server 2003 SP2              6.5.7638     

October 2005

Microsoft Exchange Server  2003 post-SP2            6.5.7653.33

March 2008

Microsoft Exchange Server 2003 post-SP2           6.5.7654.4

August 2008  

Microsoft Exchange Server 2007                    8.0.685.24 or 8.0.685.25

December 2006

Microsoft Exchange Server 2007 SP1                8.1.0240.006

November 2007

Microsoft Exchange Server 2007 SP2                8.2.0176.002

August 2009

Microsoft Exchange Server 2007 SP3                 8.3.0083.006

June  2010

Microsoft Exchange Server 2010                    14.00.0639.021

October 2009

Microsoft Exchange Server 2010 SP1               14.01.0218.015

August 2010

Ref: http://support.microsoft.com/kb/158530