3.7. DOMAINSDomains are a recently introduced concept in the ARPA Internetmail system. The use of domains changes the address space from aflat global space of simple character string host names to ahierarchically structured rooted tree of global addresses. Thehost name is replaced by a domain and host designator which is asequence of domain element strings separated by periods with theunderstanding that the domain elements are ordered from the mostspecific 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 areused, the use of nicknames or aliases is not allowed.
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