Go to Settings > General > Email Domain. Click the button that says "New Domain."
Type in the domain URL from which you would like to send an email. Then type in the default "From name" for the emails to come from. Then type in the "From email" address that the emails will be sent from.
Click "Create" to save and create the email domain.
Your email domain will show up in Settings > General > Email Domain.
After you set up your email domain, remember to get it verified before you begin sending emails!