Just like the main domain on your account, add-on domains can have their very own email accounts. Since add-on domains does not own a separate control panel to manage their site, the email accounts have to be added through the main control panel.
Below is the steps to add email accounts for add-on domains:
1. Login to 'cPanel' with URL : yourdomainname.com/cpanel
2. Click on 'Mail'.
3. Enter the email username you would like to use for the email account and click the drop down icon to select the addon domain name.
4. Enter the desired password.
5. Click on 'Create Account'.