Large Distribution Lists (DL) cause major problems when misused. Sometimes, it's not even intentional. Someone who receives a message from the large DL and may not usually consider using the large DL (such as All Company Staff Worldwide), may do a Reply To All. 

When you have an organisation of over 10,000 users this can cause a real problem. But probably the worst offenders, are those who use Delivery and Read Reciepts. If you've ever experience a 'Message Storm' you'll know what I mean.

To secure your DL complete the following steps on all large DL's.

1. In Exchange Administrator open the Properties page for the DL and Click on the Delivery Restrictions TAB as shown below

2. Add the Mailbox's and / or DL's to the Accept messages from box.

3. Click on the Advanced TAB as shown below and un-select the Report to Message Originator box as shown below.

You should also secure the DL so it cannot be used to spam your organisation by outsiders. Add the DL to the Accept Messages from in it's own property page as shown below. This restricts it's use to it's own members.