Hi Arunkumar,
it enables Marketo to compute the result of the smart list in advance so that when the time comes, the email is sent immediately.
If your email program is planned to be sent at 10 AM today, without head start, at 10 AM, it will start computing the smart list and send the email when it's done. Is the list is having complex criteria on a large DB, it may takes minutes or tens of minutes to compute and the email will actually not be sent before this computation is over.
With head start, the computation will be done a few hours in advance so that everything is ready and the email can be shot at 10:00
-Greg
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