To create a new mailbox, choose the domain from the Drop-Down-Menu. Enter the localpart of the E-mail address (the part before @) into the next input box.
There should be an addressee "postmaster" and "info" for every domain. In view of the spam problematic you should refrain from setting up a Catch-All-Forwarder.
You have to define a password for E-mail transfer. Please take a look at our advice for secure passwords.
After creation you will receive a username which contains 8 characters. Configure your E-Mail program appropriately, i.e. enter this username into the field "Username" or "User ID" oder "LoginID".
The mailserver for incoming and outgoing E-mail is mail.your-domain.com.