Exchange data fragmentation occurs within the database. To reduce its effect, Exchange Server includes a default defragmentation process (called online defragmentation) on daily basis as a part of regular maintenance process. Also, the application provides an offline defrafgmentation technique, which is accomplished by eseutil /d command. Both of these methods discard the unused space, but occasionally introduce some database errors that cause database to corrupt. To repair such corrupted databases, you need to apply eseutil repair commands or restore from the last available backup. However, if these measures don't help, you should repair the corrupted database using an Exchange Recovery tool.

For instance, consider the following practical scenario. You try to defragment the Exchange database (*.edb) with eseutil /d command, but the process fails while it is defragmenting the database. After this, you observe that the file in effect is named as *.edb.edb. You rename it to *.edb, but Exchange Server refuses to mount it further...