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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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Figure 15-2 illustrates the normal transmission of a single message from a client<br />

to a server. Additionally, we provide a textual scenario in Figure 15-3 on<br />

page 565.<br />

Figure 15-2 SMTP: Normal SMTP data flow - One mail message to one destination<br />

mailbox<br />

In the previous description, we mentioned only the most important comm<strong>and</strong>s.<br />

All of these are comm<strong>and</strong>s that must be recognized in each SMTP<br />

implementation. Other comm<strong>and</strong>s exist, but are optional. RFC st<strong>and</strong>ards do not<br />

require that every SMTP entity implement them. However, they provide very<br />

interesting functions such as relaying, forwarding, mailing lists, <strong>and</strong> so on. Some<br />

of these extensions can be referenced in Table 15-1 on page 557. For a full list of<br />

comm<strong>and</strong> verbs, refer to RFC 2821.<br />

564 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong>

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