If an Exchange Server database doesn't undergo proper shutdown, it remains attached to its associated transaction log stream. Exchange identifies this state of database as 'Dirty Shutdown'. This simply means all the data from the transaction log stream has not securely committed to the database files. You can examine the database state using eseutil/ mh command that examines the database header information. At next startup, Exchange detects this problem and applies the required logs to the database. In this situation if you remove the transaction logs, the database loses the ability to mount and cannot be restored unless you apply a backup or use some Exchange Server Repair utilities to repair it.

As one of the practical problems of such problems, you may find an Exchange database failing to mount. Also, you receive an error that reads similar to below:

Operation terminated with error -550...