Exchange Server Error Code 550
This error can occur when running eseutil.exe or when trying to start
the Microsoft Exchange Directory service. (Exchange
Recovery Clinic highly discourages the use of eseutil.exe because many
times these utilities can create more problems and destroy data.)
You should be conducting regular maintenance and backup procedures with the utilities in the Microsoft Exchange Server Manager or a third party Exchange Agent. If you are at the point where you have to use eseutil or isinteg, then you should call a Exchange Server Recovery Specialist.
If this error appeared when running eseutil.exe on one of the information stores, you could still have the store mounted. If you do not have the information store mounted, then it is more than likely that the stores are corrupted and you should restore from a recent backup if possible. If you don't have a backup or your backup is out of date,Exchange Recovery clinic can still recover your mailboxes from the stores and either create pst files, put them into a new information store,or repair your information store so you can mount it on your existing exchange server.