Email messages are queued until the message store is available. You can see the queue building if you look at the SMTP protocol queue in Exchange System Manager. They are held in the C:\Program Files\Exchsrvr\Mailroot\vsi 1\queue folder on the hard drive of the exchange server.
Performing defragmentation don’t actually cause a loss of file or data. It just basically arrange the cluttered files stored from the different parts of the hard disk drive to make a faster loading of the files.