To maximize deliverability and protect the security of our customers, we require you to verify your sending domain. This will also configure your authentication settings like SPF and DKIM to prove to mailbox providers your legitimacy as a sender, greatly improving overall deliverability and security.
Once your domain is verified, you may send email on behalf of your personal sending domain (including any subdomains).
For more info, please visit our product guide on Sender Verification.