To use MultipleWordpress to send email, you need install the smtp plugin from your wordpress site first. For example install Easy WP SMTP
Please follow the steps below to install it.
1. Login to your wordpress site -->My sites-->Network Admin->plugins panel.
2. Search for WP-Mail-SMTP
and install it
3. Active it
Now you can use it to set SMTP authentication.
1. Go to My Sites->My Wordpress->Dashboard panel.
2. Click Settings->Email for the 'Advanced Email Options'
Note: username should be same as From Email.