You should always avoid making your Microsoft Exchange Server computer a PDC. If this computer becomes unavailable, an alternate domain controller must be promoted to become the primary domain controller. If the Microsoft Exchange Server computer is not the PDC, you do not need to worry about promotions and demotions of domain controllers in a recovery situation.
Some companies prefer to place the Microsoft Exchange Server on a BDC in the accounts domain so that a second computer is not required for Windows NT authentication in remote offices. This can save the cost of purchasing another computer. However, make sure to account for additional RAM overhead for the Windows NT SAM, in addition to Microsoft Exchange Server memory requirements. Windows NT domain controllers require RAM equal to 2.5 times the size of the SAM.
If the Microsoft Exchange Server computer is a member server and not a PDC or BDC, additional memory overhead for the domain SAM is not required. However, for remote offices, companies can save money by using the local Microsoft Exchange Server to provide authentication (BDC) and messaging services.
Note For a proper directory service restore, access to the original SAM is required. Never install a Microsoft Exchange Server computer in a domain that does not have a BDC.
An alternative is to place the Microsoft Exchange Server computers in a large resource domain that trusts each account's domain. In this case, the Microsoft Exchange servers can be placed on BDCs without incurring significant memory overhead because the SAM for the Microsoft Exchange Server resource domain will be relatively small in size.