Exchange server 2003 and BDC

Exchange 2003 requires Active Directory. Active Directory doesn't use PDCs and BDCs as such, it uses 5 different operations master roles - this has to be assumed unless you're going to fill in a little detail about your specific set-up.

Its advisable to distribute these roles between servers to prevent total drop out due to DC failure.

Without those roles present on the network things like creating accounts, changing accounts etc will not work.

However, the other domain controllers will still contain a copy of Active Directory and all its associated accounts and permissions, they can always sieze those roles in the event of complete DC failure.

The decline in performanace whatever happens in this situation is rarely instantaneous, but you will encounter problems if it's left running without those roles for a few days / weeks.

Exchange will complain in the event log if there is a problem with it accessing the domain, but it won't really send everything is fine type messages.