An email domain is the part of the email address after the at (@) sign (e.g., yahoo.com in the U.S., and yahoo.ca in Canada).

You can only get an extra mail address for the same email domain as your primary mail address. So if your mail address is joe@yahoo.de (Joe - ? You're in Germany?) then you could not set up an extra email address such as joe.smith@yahoo.com. You'd have to pick an address that ends with @yahoo.de.