How do I send email notifications from a custom email address?

You can configure your store to send emails from your own domain.

Go to Settings > Customizations > Email Notifications > Sender Email Preferences and click + New Sender.
Add your name and email address, then verify it via the verification email sent to that address.
To ensure emails reach the inbox and are not flagged as spam, authenticate your domain by clicking Authenticate Now next to your domain.
Update the DKIM records (Host Name and Value) in your domain provider’s DNS settings.
Once authenticated, your custom email address can be used for sending transactional notifications.