Type a user name before @ in the E-mail field, and select one of your domains. If you want to create mailboxes for this user in each of your domains, select (multiple domains).
Type a password for a user to read his (or her) mail. Finally, you can optionally add a free-form comment, just for future references.
To add a mailbox, click "Finish".
If you want to configure an autoresponder for this mailbox right away click the "Next" button.