What is the difference between Exchange Server APM and Exchange Mailbox?
The Exchange Server APM backups all of Exchange as a whole.
It backs up the Public Folders and Mailstore. The backup is faster than the Mailbox APM was, since it is backing up the whole database.
The Exchange Mailbox APM used mapi calls to backup the Public Folders and Mailboxes. It was slower, and a more granulated backup. This plugin was decomissioned in 2008.