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...