USMTP
USMTP awakens periodically (or when signaled by SPOOL#4 or SNDMSG) to try to
send pending mail data sets. It tries to send each data set that is either new
or whose retry timeout has expired. Any data sets successfully sent are deleted
and uncataloged. The others are kept for later attempts. USMTP expects the mail
data sets to be in the format shown in the example in the section on
Sending Mail. USMTP assumes only a single X-TO: line (in other words,
recipient mail address) per data set. For multiple network recipients, the post
office (or SNDMSG) must create multiple data sets.
USMTP generally handles arbitrary carriage control
characters in the mail data set translating them to corresponding sequences of
ASCII format effectors.
If transmission of a particular message is unsuccessful
because of a temporary error (for example, the remote host is down), USMTP
retries periodically for a limited number of times. If this number is exceeded,
or there is a permanent error (for example, the remote SMTP receiver refused the
message because the target local part of the mail address was unknown), USMTP
redelivers the message to the local user. For this redelivery, USMTP issues a
message from the Mailer Daemon saying that the message was undeliverable and
why. USMTP appends the Daemon message to the original message and spools it into
the JES queue used by SSMTP (see the section of this chapter describing SSMTP)
to deliver messages to the post office.
USMTP configuration parameters determine the frequency of
retries and the maximum retry time.
USMTP has the following general features:
-
- >To provide robust delivery, USMTP tries each address in
turn of a multi-homed host before deciding that the mail cannot be
delivered.
-
- USMTP scans the fields in the message header and
envelope that contain host names and, if necessary, maps nicknames into
official host names.
-
- USMTP uses this feature to resolve mail routes for a
host. (similar to a DNRGET ROUTE BYNAME host_name).
-
- As mentioned, USMTP scans the mail addresses in the
envelope and message header and, if possible, uses the SMTP User Table to
map local TSO user IDs into a mailbox name string before sending the
message.
Note To use Host name mapping and MX record
support, USMTP must be able to parse RFC 822 message headers.
- File Inclusion
-
- A message line beginning with the three characters ..f=
followed by a data set name causes USMTP to include the contents of that
data set in the outgoing message. Do not enclose the data set name in
quotes; it must be fully qualified. This is an example:
- Because included files are now expanded by SNDMSG,
USMTP normally does not see ..f= commands. Such commands may have been
entered by some other mailer via SPOOL#4.
- Support for Domain Literals
- You can send a message using a domain literal in place
of a host name. However, since it is difficult to enter square brackets on
an IBM system, USMTP accepts IP address domain literals without brackets:
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