Mailbox Manager has been around since Exchange 5.5 Service Pack 3. You can use Mailbox Manager to enforce message retention policies and mailbox storage limits, and many organizations depend on it.
However, the Exchange 2000 version of Mailbox Manager has an acknowledged bug. If you schedule Mailbox Manager for one 15-minute block, it may not run if your Exchange server is under load. Unfortunately, this behavior isn't very predictable or repeatable.
In most cases, this flaw is undesirable, but it isn't fatal. If Mailbox Manager skips a scheduled run every now and then, most IT departments can live with it.
For more severe cases, there's a workaround you can try. Simply scheduling two 15-minute blocks of time increases the chances that at least one of them will run.
But even this workaround is no guarantee that a heavily loaded server will run Mailbox Manager consistently. In addition, there's the possibility that Mailbox Manager will run twice, which can produce undesired results in some cases. (For more information, see Microsoft Knowledge Base article 285695.)