"In what way do Exchange and the PDC Emulator relate?" Well, before starting this topic you should know what the PDC Emulator is.

The PDC Emulator is one of the 5 Active Directory FSMO Roles (in Full - "Flexible Single Master Operation Roles"). The others are Schema Master, Domaining Naming master, Infrastructure Master and the RID Master.

What are the duties of the PDC Emulator:

• The PDC Emulator emulates a Windows NT 4.0 Primary Domain Controller - so that NT 4 Backup Domain Controllers can successfully replicate Directory Information (of course, in a pure Win2k/2k3 enviroment, this feature becomes obsolete).
  
  
• When a User changes his/her password on another DC, the password change is preferentially replicated to the PDC Emulator before any other DC.
  
  
• Furthermore, if a user tries to log on to a DC and the DC didn't receive the password change it will forward the logon request to the PDC Emulator (as mentioned before, the PDC Emulator gets preferential treatment on password changes).
  
  
• Account lockouts are processed by the PDC Emulator.
  
  
• When modifying or creating GPO's, this is always done on the SYSVOL - Share of the PDC Emulator.
  
  
• The PDC Emulator is the authoritive source of time in the domain - all other servers and clients will try to obtain time information from the PDC Emulator (for that reason, the PDC Emulator should always be configured with an external accurate time source).
  
  

So the PDC Emulator is a pretty important role within a domain. But what's up with Exchange? More...

I'am pretty sure you saw it before: User xy calls, his mailbox is over his size limit, but he must send or receive this one important piece of mail just now. You head to Active Directory, change the mailbox size limits. The User tries again, still not working.

You take a look into Exchange System Manager, and see that the new values have not been read by Exchange.

So what happens here?

Exchange 2000/2003 uses cached mailbox configuration data to determine the enforced mailbox size limits. By default, the cache has a TTL of 2 hours, meaning it can take as much as two hours for you changed size limits to be enforced. More...