These options will let you control how mail is sent and received.
You can restrict the maximum size of messages when they are received.
This feature is intended for people without continuous Internet connectivity. This tells sendmail to save the mail without further checking. Later on, one can trigger the delivery of the messages. This is usually done by specifying a post connection command (see Linuxconf ppp dialout support) like "/usr/sbin/sendmail -q". This forces sendmail to process its queue and deliver the messages.
In many cases, you may prefer to set the mail gateway protocol to expensive ESMTP, which queues outgoing mail, but processes local mail.
This setting will instruct sendmail to process queued mail. Sendmail will wake up and try to deliver email that it was unable to deliver previously. You can see the content of the queue with the command "mailq" (without arguments).
People without continuous Internet connectivity may want to play with this value. Setting the delay to 0 disables the feature. If your connectivity is on-demand, you may want to enter a larger number, such as 30 minutes. Then, when sendmail wakes up, it will/should trigger a connection to the Internet and process any queued mail.
Those who disable this feature may want to control sendmail manually (or with a cron job) with the command:
/usr/sbin/sendmail -q
Users are allowed to encode some processing instructions in their
.forward file. The processing is done with the bourne shell
(/bin/sh). Using this feature, processing is done within a
"restricted" shell. This shell only executes commands found in
the /etc/smrsh directory.
A message may include several recipients and you may want to restrict the maximum number.
Sendmail makes heavy use of the DNS. Machines with no Internet connectivity at all (even behind a firewall) may want to deactivate DNS usage.
Alternately, machines with normal DNS connectivity should force DNS usage.