Exchange server has 4 types of mail boxes :
user mail box, Room mail box, Equipment mail box, Linked mail box.

Its is an active dirctory object but not owned by any user and its used for creating conference, training rooms etc...

A DAG is the base component of Exchange 2010 high availability and provides a boundary for mailbox replication, servers, and failovers. A DAG can span Active Directory sites to provide intrasite and intersite redundancy.

It has following features :---

• A DAG supports up to 16 nodes.
• All Exchange servers must be in the same domain.
• Only servers in the same DAG can be replication partners.
• DAG members can be in different Active Directory sites.
• DAG members must have the same operating system level ( NN Windows Server 2008 or Windows Server 2008 R2)
• DAG members require Windows 2008/2008 R2 Enterprise Edition or Datacenter Edition.
• You cannot install Windows NLB and Windows Failover Cluster on the same server.
• Replication occurs at the database level, allowing for a single database to be moved.

The tombstone lifetime in an Active Directory forest determines how long a deleted object - aka a 'tombstone' - is retained in Active Directory. The tombstone lifetime is determined by the value of the tombstone Lifetime attribute on the Directory Service object in the configuration directory partition.
Tombstone Lifetime assists in removing objects from replicated servers and preventing restores from reintroducing a deleted object. Actually when an object is deleted from Active Directory, it is not physically removed from the Active Directory for some days. Instead, the Active Directory sets the 'isDeleted' attribute of the deleted object to TRUE and move it to a special container called 'Tombstone'.