Goto Settings → Email → Domain → Add Domain
Add your owned Domain Name
Verify Your Domain
Goto Settings → Email → Domain → domain name
Copy SPF, DKIM & Tracking record values of your domain
Go to your domain hosting website's Settings page and paste the copied record values and Publish them to the DNS servers.
After your records are published, Goto Settings → Email → Domain
Click on Verify Button
After Domain verification your account is ready to start sending.
Add Sender ( After Domain Verification )
Goto Settings → Email → Sender → Create New Sender
Click on Submit Button
Generate API Key
Goto Transactional Emails → API
Click on Enable API Access Button.
Now Use newly generated username and api_key in transactional emails api.