The big question with backups always is "What should we back up?"

There are many reasons that contribute to your final decision. Of course, you want to back up your server's configuration because it is essential to your server's functionality. But you also want to back up the users' data because it is your business' valuable assets. Is there a company policy that says people may use e-mail for private communication? If there is, will you back up those messages as well?

You should only back up what you need to restore your system to a functional state. This saves space on your backup media and shortens the time required to perform a backup.

After all, the space on your backup media is limited and thus precious. It is more important to back up all the users' mail than to have a complete backup of your /tmp directory. Also, the less data you back up, the less time is required to perform the backup, thus returning your system's resources (CPU cycles, I/O bandwidth) faster to their...