As the new version of Exchange is coming, I looked up the release dates of all the previous versions. Here’s the list:

Exchange 4.0 April 1996
Exchange 4.0 (a) August 1996
Exchange 4.0 SP1 May 1996
Exchange 4.0 SP2 August 1996
Exchange 4.0 SP3 November 1996
Exchange 4.0 SP4 April 1997
Exchange 4.0 SP5 May 1998
Exchange 5.0 March 1997
Exchange 5.0 SP1 June 1997
Exchange 5.0 SP2 February 1998
Exchange 5.5 November 1997
Exchange 5.5 SP1 July 1998
Exchange 5.5 SP2 December 1998
Exchange 5.5 SP3 September 1999
Exchange 5.5 SP4 November 2000
Exchange 2000 October 2000
Exchange 2000 (a) January 2001
Exchange 2000 SP1 July 2001
Exchange 2000 SP2 December 2001
Exchange 2000 SP3 August 2002
Exchange 2000 post-SP3 September 2003
Exchange 2000 post-SP3 April 2004
Exchange 2000 post-SP3 August 2004
Exchange Server 2003 October 2003
Exchange Server 2003 SP1 May 2004
Exchange Server 2003 SP2 October 2005