When you start thinking about the number of places where you can define limits for an Exchange user, you'll see that there are quite a few. However, planning for realistic and effective limits are important for your organization. Effectively limiting mailbox sizes, public folder sizes, and message sizes can help you to predict the growth of your mail storage requirements and prevent you from exceeding the storage you have available for Exchange content. Effective limits are an important part of proactive troubleshooting because they limit potential abuse of the mail system by preventing users from sending or receiving content that is too large.