To maintain optimum message flow in your Exchange organization, freezing and unfreezing messages is a useful Queue Viewer feature. Freezing one or more messages in a given queue, for example, allows your server to transport the messages that are not frozen. Once these messages have been transported, then you can unfreeze the other messages.
You can also freeze and unfreeze entire queues. When a queue is frozen it will stop transporting messages, but will continue accepting new messages. Messages inside a frozen queue are not necessarily in a frozen state themselves.
Note X.400 queues cannot be frozen.
Important Microsoft recommends freezing a queue or a group of messages before performing any administrative tasks on them. In Exchange, queues are dynamic. Therefore, if you select Delete All Messages (send NDR) on the context menu without first freezing the queue, it is possible that messages will be delivered, and thus leave the queue, before the Delete action is executed.
Similarly, when using the custom message filter it is important to freeze messages. Microsoft recommends freezing the messages you want to administrate and then selecting Frozen in the Custom Filter search criteria. That way only the messages you specifically freeze will be acted upon. Because they are frozen, they will not leave the queue before any action is taken.