Backup Exec 12.5 Exchange Datastores

I'm encountering an issue when backing up my datastores for Exchange 2007.

Whenever the backup job starts and during the "Snapshot processing" stage our Exchange server stops sending/receiving mail.

I thought the purpose of creating a snapshot and performing the backup from the snapshot was to not interfere with the operation of the excahnge server.

Circular logging is disabled on the datastores and the only way to recover from this is to restart the Exchange server.

Does anyone have any insight into this, or perhaps have experienced this before?