Ah, unintended consequences
This reminds my of my first encounter with that Exchange 2000 directory service sync tool, to bridge the gap from the days when Exchange was independent and AD didn't exist.
I remember my first migration/upgrade from 5.5 to 2000, I was a stand-in last minute consultant when my colleague was ill (or bottled it) and I was supposed to know what I was doing, the only problem been that I'd never done a 5.5 to 2000 upgrade before. I was desperately reading notes on the train, and trying to memorise the sequence. This wasn't a nice small client either, there were at least 200 people working locally, and they had 4 other sites, all with ISDN connections, remote BDC's and remote exchange servers.
Anyway, I went for it, and everything was going SO well. People were able to connect to their mailbox on the new Exchange 2000 server. Fine.
To achieve this you has to use a sync tool to hook up two products that had previously never communicated, Exchange 5.5 directory Service and this new shiny Active Directory.
In the sync tool were several options. The one most people wanted was 'create AD object for any directory service objects not in AD'. I'd guessed correct and been running that one for hours. Everything was going great. There was also a checkbox labelled "delete Exchange directory objects where there is no matching object in AD'. Having run it for several hours I couldn't see the harm in experimenting with that option, as in my mind AD now had everything that mattered. Little did I know how lethal that option is.
It instantly created two serious problems - firstly the CEO and board chairman only accessed his email using POP3 on his Psion and had no AD account, so he didn't get hooked up. Consequently his mailbox got deleted as there was no trace of it in AD.
Secondly, as I learned very fast, I hadn't yet migrated distribution groups, so running this option removed ALL distribution groups in Exchange 5.5. For all sites.
Luckily the IT department treated it as a huge joke and were able to restore the mailbox, and then phoned up every department head asking what staff they had. Somehow I kept my job.