The Internet has quite sophisticated E-mail capabilities.
Domains are hierarchical in nature and thus are extensible to any necessary level of detail.
A domain usually identifies a single computer on the Internet. For example, godzilla.zeta.org.au . But sometimes a domain does not represent an actual computer and is used as a catcher for all mail to any computer inside that domain. The domain zeta.org.au is set up that way. This protects us against needing to change our E-mail addresses as we grow, or if the computer godzilla is replaced by another in future.