For my own case, the email was sent out around two hours after the lawyer clicked the button for the perm application. My lawyer was not even aware of this issue until I told him my boss received that email. In the email, DOL required the employer to answer all questions within 7 days.
It seems DOL did not always send out that email around that time when I applied for my case. Just googled this subject online and the first link might give you some idea:
http://boards.immigrationportal.com/showthread.php?t=196458
Hope this helps and Good Luck!