Perform an offline defragmentation on Exchange database:

• Backup your Exchange database
• Dismount the database
• Open elevated command prompt and navigate to:
  C:\Program Files\Exchsrvr\bin
  or C:\Program Files\Microsoft\Exchange Server\V14\Bin
• Run: eseutil.exe /d “D:\path\database.edb” /t “D:\temp.edb”
  Where:
  “D:\path\database.edb” – path to database to defragment
  “D:\temp.edb” – path to temporary file (can be on different drive)

Microsoft Exchange Server 2007
Microsoft Exchange Server 2010