You probably have a mail group used for sending broadcast emails  (mass mailings) to many people, like all of your employees.  Do you ever have someone send a message using this group that shouldn’t?  Do you ever have someone Reply to All when they receive a message addressed this way?  Do you ever get spam addressed to that group?  The fix for this is very simple.  You can restrict access to any group by setting the reader names.  Follow these steps:

1.  Before updating your broadcast group, create a group that will be used to define who is allowed to send mail using the broadcast group.  Add the appropriate members to the group.  In many cases this group will be maintained by HR or some other person responsible for authorizing broadcast messages.

2.  Edit the group used for broadcast messages.
3.  Display the document properties of the mass mailing group.  One way to do this is press Alt-Enter, then change the object to Document.
4.  Click on the security tab (the one with the key)
5.  Unselect the field “All readers and above”
6.  Add the group created in step 1 to the list of readers.  IMPORTANT:  You MUST also add LocalDomainServers as well as your administrator group or you will not be able to view this group document once you save it.

7.  Save and close the group.

Now only people listed in those groups can send email to the broadcast group.  NO messages received via SMTP will be able to use the group because they are treated as anonymous.