Here is a simple script to change all of you exchange groups to only accept email from set people inside you domain, we found a use for this when students started to email the all staff groups from each other accounts,
Get-DistributionGroup | set-DistributionGroup -AcceptMessagesOnlyFromDLMembers "a_Group_Name"
You can also set it so only a user has access to email the group
Get-DistributionGroup | set-DistributionGroup -AcceptMessagesOnlyFrom "a_User_Name"
Or just one Group
set-DistributionGroup -Identity "a_Group_Name" -AcceptMessagesOnlyFrom "a_User_Name"