23.6.1 Periodically with -q

The -q command-line switch is used both to cause the queue to be processed and to specify the interval between queue runs.

A typical invocation of the sendmail daemon looks like this:

/usr/lib/sendmail -bd -q1h

Here, the sendmail program is placed into listening mode with the -bd command-line switch. The -q1h command-line switch tells it to process the queue once each hour. Note that either switch puts sendmail into the background as a daemon. The -bd switch just allows sendmail to listen for incoming SMTP connections. Consider the following:

/usr/lib/sendmail -bd
/usr/lib/sendmail -q1h

This runs two daemons simultaneously. The first listens for incoming SMTP connections. The second processes the queue once per hour.

At small sites, where mail messages are rarely queued, the time interval chosen may be small, to ensure that all mail is delivered promptly. An interval of 15m (15 minutes) may be appropriate.

At many sites an interval of one hour is probably best. It is short enough to ensure that delays in delivery remain tolerable, yet long enough to ensure that queue processing does not overlap.

At large sites with huge amounts of mail and at sites that send a great deal of international mail, the interval has to be carefully tuned by observing how long it takes sendmail to process the queue and what causes that process to take a long time. Points to consider are the following:

• Network delays or delays at the receiving host may cause delivery to that host to time out. Timeouts are set with the Timeout ( r ) option (see Section 34.8.70 ). [5] Each such timeout is logged at LOG_NOTICE with a message like this:

  [5] Note that prior to V8 sendmail the r option set one timeout for all SMTP timeouts.

  timeout waiting for input from 
  
  host
  
   during 
  
  what
  
  

  Here, host is the name of the other host, and what specifies which timeout triggered the message (such as "client HELO for to_helo ). In general, timeouts should be large to ensure that mail to busy sites and to large mailing lists does not time out improperly. In observing the queue processing, you may find that all messages but one process swiftly. That one, you may find, takes over an hour because of a long SMTP timeout. A possible solution to this problem is to make all timeouts short so that most queue runs are processed quickly. Then, for example, the following command could be run a few times each night to specifically flush those long jobs:

  /usr/lib/sendmail -or2h -q

• The queue can take a long time to process because too many messages are being queued unnecessarily. Several options affect the placement of mail messages into the queue. The QueueLA ( x ) option (see Section 34.8.50 ) tells sendmail to queue, rather than deliver, a message if the machine load is too high. Fewer messages will be queued if the value of that option is increased. The SuperSafe ( s ) option (see Section 34.8.67, SuperSafe (s) ) tells sendmail to queue all messages for safety. If your machine "never" crashes, this may not be necessary. [6] The HoldExpensive ( c ) option (see Section 34.8.29, HoldExpensive (c) ) tells sendmail to queue messages to "expensive" delivery agents (those with the F=e flag set; see Section 30.8.18, F=e ) rather than delivering them. If the queue is routinely filled with messages to expensive sites, you should reconsider your reasons for marking those sites as expensive.

  [6] Eric says, " Set this option . In fact, it may go away (or be ignored, i.e., always be on) in some future release."

• The queue can fill with messages because sendmail was run with the -odq or -odd command-line switch (see the DeliveryMode ( d ) option, Section 34.8.16 ). At sites that receive a great deal of UUCP mail for forwarding, the rmail (8) program is often set up to run sendmail in "queue only" mode with the -odq command-line switch. If UUCP mail is clogging your normal mail services, you should consider queueing it to a separate queue directory. You can then process that other directory with a separate queue run of sendmail . (Use of separate queue directories is discussed in Section 23.7, "Process Alternate Queues" .)

• A slow machine can clog the queue. When a single machine is set up to handle the bulk of a site's mail, that machine should be as swift as possible. In general, a dedicated mail server should have a fast CPU with lots of memory. It should never allow users to log in to it, and it should run its own name server daemon.

23.6.2 From the Command Line

The -q command-line switch, invoked without a time interval argument, is used to run sendmail in queue-processing mode. In this mode, sendmail processes the queue once and then exits. This mode can be run interactively from the command-line or in the background via cron (8).

Other command-line switches can be combined with -q to refine the way the queue is processed. The -v (verbose) switch causes sendmail to print information about each message it is processing. The -d (debugging) switch may be used to produce additional information about the queue. We'll discuss the -v switch as it applies to the queue later in this chapter. Those -d debugging switches appropriate to the queue can be found in Table 37.2 of Section 37.5, "Reference in Numerical Order" .

V8 sendmail allows variations on -q : -qI allows you to specify a specific message identifier for processing; -qR allows you to specify specific recipient addresses for processing; -qS allows you to specify specific sender addresses for processing. [7]

[7] IDA and Pre-V8 SunOS sendmail offer three different command-line switches for processing the queue. The -M switch allows you to specify a specific message for processing. The -R switch allows you to specify specific recipient addresses for processing. The -S switch allows you to specify specific sender addresses for processing.