The solution is simple: Exchange has a "Self protect" feature by recognising if it has been backed up or not. If it detects that the database isnt properly backed up either using NTBackup the SBS Backup or another body which correctly cleans the database then the exchange will begin logging every email in plain text into a Text document in the C:\Progr~\Exchsrv\MDBDATA directory.
So the solutions are simple if you are using a backup program then make sure it is running ok. if you are using your own custom built backup script like Xcopy or something like, you may need to look at Circular Logging to stop the build up of log files.
The only other reason I have so far found that this can happen however are Database corruption; to check this again is relatively simple if you have used the ESEUtil application before.
Open a command prompt and navigate to C:\Progr~\Exchsrv\Bin\
To repair look at following the below articles:
How to remove Transaction logs
How to Maintain after repair