Mac OS X Lion has a feature which can apply restriction for Mail and Chat apps based on the contacts. For example, you can use this to allow your kids to exchange emails or have chat conversation with only certain contacts.
Click the Apple icon, select System Preferences from the menu list.
In the System Preferences window, click the Parental Controls option available under System section.
Make sure to unlock the Parental Control preferences for editing by clicking the “Click the lock to make changes”
Now select the user for which you want to apply the access restriction then click the People tab.
If you want to apply restriction for Mail and Chat apps then mark both the check boxes with caption as “Limit Mail” and “Limit iChat“. Then enter the accepted contacts by using the + and – available below the Allowed Contacts section.
There is also an another feature which would send an email to the specified address whenever the user attempts to exchange email with the contact who is not in the approved list.
You can enable this feature by marking the check box “Send permission requests to” then entering the email address. This is quite useful when you have too many allowed contacts list and you just want to monitor the email and chat activity of your kids.