Domain Names

A domain name (or often just a "domain") consists of one or more

components, separated by dots if more than one appears. In the case

of a top-level domain used by itself in an email address, a single

string is used without any dots. This makes the requirement,

described in more detail below, that only fully-qualified domain

names appear in SMTP transactions on the public Internet,

particularly important where top-level domains are involved. These

components ("labels" in DNS terminology, RFC 1035 [2]) are restricted

for SMTP purposes to consist of a sequence of letters, digits, and

hyphens drawn from the ASCII character set [6]. Domain names are

used as names of hosts and of other entities in the domain name

hierarchy. For example, a domain may refer to an alias (label of a

CNAME RR) or the label of Mail eXchanger records to be used to

deliver mail instead of representing a host name. See RFC 1035 [2]

and Section 5 of this specification.

The domain name, as described in this document and in RFC 1035 [2],

is the entire, fully-qualified name (often referred to as an "FQDN").

A domain name that is not in FQDN form is no more than a local alias.

Local aliases MUST NOT appear in any SMTP transaction.

Only resolvable, fully-qualified domain names (FQDNs) are permitted

when domain names are used in SMTP. In other words, names that can

be resolved to MX RRs or address (i.e., A or AAAA) RRs (as discussed

in Section 5) are permitted, as are CNAME RRs whose targets can be

resolved, in turn, to MX or address RRs. Local nicknames or

unqualified names MUST NOT be used. There are two exceptions to the

rule requiring FQDNs:

o The domain name given in the EHLO command MUST be either a primary

host name (a domain name that resolves to an address RR) or, if

the host has no name, an address literal, as described in

Section 4.1.3 and discussed further in the EHLO discussion of

Section 4.1.4.

o The reserved mailbox name "postmaster" may be used in a RCPT

command without domain qualification (see Section 4.1.1.3) and

MUST be accepted if so used.

Buffer and State Table

SMTP sessions are stateful, with both parties carefully maintaining a

common view of the current state. In this document, we model this

state by a virtual "buffer" and a "state table" on the server that

may be used by the client to, for example, "clear the buffer" or

"reset the state table", causing the information in the buffer to be

discarded and the state to be returned to some previous state.