Each version of Microsoft Exchange Server has a different build number. The following table lists the build numbers and general release dates for each version.

Version                                             Build number              Release date 
---------------------------------------------------------------------------------------------- 
Microsoft Exchange Server  4.0                      4.0.837                   April 1996 
Microsoft Exchange Server  4.0 (a)                  4.0.993                   August 1996 
Microsoft Exchange Server  4.0 SP1                  4.0.838                   May 1996 
Microsoft Exchange Server  4.0 SP2                  4.0.993                   August 1996 
Microsoft Exchange Server  4.0 SP3                  4.0.994                   November 1996 
Microsoft Exchange Server  4.0 SP4                  4.0.995                   April 1997 
Microsoft Exchange Server  4.0 SP5                  4.0.996                   May 1998
 
Microsoft Exchange Server  5.0                      5.0.1457                  March 1997 
Microsoft Exchange Server  5.0 SP1                  5.0.1458                  June 1997 
Microsoft Exchange Server  5.0 SP2                  5.0.1460                  February 1998 

Microsoft Exchange Server  5.5                      5.5.1960                  November 1997 
Microsoft Exchange Server  5.5 SP1                  5.5.2232                  July 1998 
Microsoft Exchange Server  5.5 SP2                  5.5.2448                  December 1998 
Microsoft Exchange Server  5.5 SP3                  5.5.2650                  September 1999 
Microsoft Exchange Server  5.5 SP4                  5.5.2653                  November 2000
 
Microsoft Exchange 2000 Server                      6.0.4417                  October 2000 
Microsoft Exchange 2000 Server (a)                  6.0.4417                  January 2001 
Microsoft Exchange 2000 Server SP1                  6.0.4712                  July 2001 
Microsoft Exchange 2000 Server SP2                  6.0.5762                  December 2001 
Microsoft Exchange 2000 Server SP3                  6.0.6249                  August 2002 
Microsoft Exchange 2000 Server post-SP3             6.0.6487                  September 2003 
Microsoft Exchange 2000 Server post-SP3             6.0.6556                  April 2004 
Microsoft Exchange 2000 Server post-SP3             6.0.6603                  August 2004
 
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  2007                     8.0.685.24 or 8.0.685.25  December 2006 
Microsoft Exchange Server  2007 SP1                 8.1.0240.006              November 2007

To view the build number of the server that you are using, see the properties of the server.