http://blogs.technet.com/exchange/archive/2004/06/30/169953.aspxThe msExchPreviousAccountSid is used to identify when there was a problem with connection agreements being rolled out before the 5.5 directory is properly prepared. It is the SID the ADC wanted to use as the msExchMasterAccountSid but couldn't becauseen-USCommunityServer 2.1 SP1 (Build: 61025.2)