Introducing SMTP
The SMTP is used as the common mechanism for transporting electronic mail
among different hosts within the Department of Defense Internet protocol suite.
Under SMTP, a user SMTP process opens a TCP connection to a
server SMTP process on a remote host and attempts to send mail across the
connection. The server SMTP listens for a TCP connection on a well-known port
(25), and the user SMTP process initiates a connection on that port. When the
TCP connection is successful, the two processes execute a simple
request/response dialogue, defined by the SMTP protocol, in which the user
process transmits the mail addresses of the originator and the recipient(s) for
a message. When the server process accepts these mail addresses, the user
process transmits the message. The message must contain a message header and
message text formatted in accordance with RFC 822.
Mail Transport Support Programs
Within Cisco IOS for S/390, these major programs support
mail transport:
The SMTP Server program receives SMTP mail and spools it into a
JES
SYSOUT file.
The SMTP User program assumes that mail to be sent is
available in cataloged data sets with a specific DSNAME prefix. It awakens
periodically (or in response to a signal from SPOOL#4 or SNDMSG) to try to
send pending mail data sets.
The outgoing mail spooler copies output from a JES print
queue into cataloged data sets that are the outgoing mail data sets for
USMTP. SPOOL#4 then awakens USMTP to send the messages.
This interactive program lets local users create
outgoing mail files. Using data entered from a local user terminal, SNDMSG
creates mail data sets in the same format as SPOOL#4. SNDMSG then awakens
USMTP to send the messages.
Cisco IOS for S/390 contains no mechanism to read the JES
queue containing received mail (the queue written by SSMTP) nor to write the JES
queue with mail to be transported (the queue read by SPOOL#4). To provide these
mail services, an installation needs an appropriate user mail system. USMTP,
SSMTP, and SPOOL#4 interface through JES2/NJE with a user mail system such as
UCLAMAIL. However, since the interfaces are quite general, some other mail
system can be substituted. >
In this chapter, the user mail system is referred to as the
post office, meaning either UCLAMAIL or some other mail system. Read
Interface to a User Mail System for a description of the interfaces between the
Cisco IOS for S/390 mail routines and the post office.
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