@ Indunil Jayasooriya
IIRC, in one of your follow up post, you mention that you have the Zimbra suite installed and running on *this* machine (which is why you might have had to remove the distro's default postfix in the first place).
If above is true then you should try to figure out how to send the cron emails using Zimbra's MTA (which BTW is also postfix).
Why are you trying to re-invent the wheel?
Am I missing something here?