Delivery mode
There are a number of delivery modes that
sendmail(1M)
can operate in, set by the DeliveryMode configuration
option. These modes specify how quickly mail will be delivered. Legal
modes are:
i-
deliver interactively (synchronously)
b-
deliver in background (asynchronously)
q-
queue only (don't deliver)
d-
defer delivery attempts (don't deliver)
There are tradeoffs. Mode i gives the sender the quickest
feedback, but may slow down some mailers and is hardly ever necessary.
Mode b delivers promptly but can cause large numbers of
processes if you have a mailer that takes a long time to deliver a
message. Mode q minimizes the load on your machine, but means
that delivery may be delayed for up to the queue interval. Mode
d is identical to mode q except that it also prevents
all the early map lookups from working; it is intended for dial on
demand sites where DNS lookups might cost real money.
Some simple error messages (for example, host unknown during the
SMTP protocol) will be delayed using this mode. Mode
b is the usual default.
If you run in mode q (queue only), d (defer), or
b (deliver in background), sendmail will not expand
aliases and follow .forward files upon initial receipt of
the mail. This speeds up the response to RCPT commands.
Mode i cannot be used by the SMTP server.
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