[Retrieving past send items from MS Exchange Server]

my employer is currently cracking down on inappropriate emails and has been, i think, retrieving emails from the server that have been sent in the past. These are then printed and used as evidence that the employee has been sending stuff that shouldn't be sent.

Everyone is worried since i think most people have sent something they shouldn't at some time or another - some more often/serious than others.

My question is, what exactly can be retrieved from the exchange server using this method? Is every single email completely archived, including attachments? or is it simply a log file with the header displayed?
I would have thought that to store every email for a large organisation would incur massive maintenance and back up costs and therefore unlikely or impractical. I reckon complete email archiving would require several gigabytes per day.

Does anyone know what can be retrieved from the server, and how far back emails are/can be stored?

thanks