It was later decided that other domains could also be used as TLD. Some TLD were reserved in RFC 2606:

Domains that are not directly connected to the Internet can also exist, i.e., computers that do not even use the TCP/IP network protocol therefore do not have an IP address. These domains are sometimes called pseudodomains. They are meaningful especially for electronic mail. It is possible to send an email into other networks and then into the Internet with the help of a pseudodomain (like DECnet or MS Exchange).

In its internal network, a company can first use TCP/IP and then DECnet protocol. A user using TCP/IP in the internal network (for example,< [email protected]>) is addressed from the Internet. But how do you address a user on computers working in the DECnet protocol?

To solve this, we insert the fictive dnet pseudodomain into the address. The user Daniel is therefore addressed [email protected]. With the help of DNS, the entire email that was addressed into the dnet.company.com domain is redirected to a gateway in DECnet protocol (the gateway of the company.com domain), which performs the transformation from TCP/IP (for SMTP) into DECnet (for Mail-11).