Domains are a recently introduced concept in the ARPA Internet mail system. The use of domains changes the address space from a flat global space of simple character string host names to a hierarchically structured rooted tree of global addresses. The host name is replaced by a domain and host designator which is a sequence of domain element strings separated by periods with the understanding that the domain elements are ordered from the most specific to the most general.
For example, "USC-ISIF.ARPA", "Fred.Cambridge.UK", and "PC7.LCS.MIT.ARPA" might be host-and-domain identifiers.
Whenever domain names are used in SMTP only the official names are used, the use of nicknames or aliases is not allowed.