I have generated the coupon codes (Promotions > Shopping Cart Price Rule). I am not getting the way, how to send these coupons on customer's mail id's.
It's not built into magento to mass send coupon codes or even individually send coupon codes.
You would want to use something like mailchimp that supports mail merge to send dynamic codes to each customer on your list individually. You would be able to upload a csv of codes to mailchimp for it to grab from.
If you are dead set on using magento native newsletter feature some custom coding will be required. There are also numerous plugins that support this functionality such as Aheadworks, Mirasvit, and Amasty newsletter or follow up email extensions.