To send your emails you have to add a sender email address or domain to your API key and validate it. You can do so from the Mailjet app or via the API.
In addition, you will need to modify your DNS records to include DKIM signature and SPF. This will help Inbox Service Providers (ISPs) and webmail servers (e.g. Gmail or Yahoo) verify that the emails are being sent from an authorized server. Once setup, SPF & DKIM will also help with your deliverability – meaning your emails have a better chance of being delivered to the recipient’s inbox and not their spam folder.