If you don't want to use DPM Disk to Disk to Tape (D2D2T) solution and instead want to utilize a 3rd party backup product for long term offsite tape backups - Using 3rd party to backup a DPM server is described here: http://technet.microsoft.com/en-us/library/bb809019.aspx
However, restores of protected data will need to be performed to an alternate location (Not back to the DPM server) by the 3rd party application.
What I mean by this is the 3rd party backup application will be protecting the DPM replica volumes (either directly, or via a snapshot using dpmbackup replicas) and if a restore is required of a data that is older than what DPM has on disk, then the restore from tape will need to restore the files to an alternate location anywhere but the DPM replica, as that will compromise current DPM disk based backups.
You can then manually mount the exchange database and use exchange tools to extract mailbox data.
Alternate solution would be to hardware partition the tape library and let Netbackup use one partition and DPM the other so you can do real D2D2T backup with DPM for the 7 year retention.
Regardless of which tape backup solution you choose, it will need to be full backup. If you let DPM do it, it mounts the last express full disk based recovery point and does a full backup to tape. If you use 3rd party, you first run dpmbackup -replica - then perform a full backup from the shadowcopy folder.
If you restore from DPM tape backup, it will restore the DB to an exchange recovery storage group (RSG) (or overwrite the current DB if that is what you choose) and bring the DB into a consistent state. If you restore from 3rd party, you need to restore as flat files, then manually run eseutil.exe to replay the logs and get the DB into consistent state, then attach it to the RSG, or overwrite existing DB.
Yes, that is correct. DPM will have no knowledge of the files restored from 3rd party tape. It's strictly a manual process if you are not using DPM for long term tape backup.