With the Send API you will not be able to use contact lists or segmentation, but instead will have to provide the recipients your API calls. This option is useful when you have a separate platform that handles your contacts and segmentation options outside of Mailjet.
The Send API also allows you to personalize the message content for your recipients to a great extent, since you are not required to send the same template / email content to all users.
The sender email address you will be using should be validated in your Mailjet account - see the 'Senders and Domains' guide for more info.