Mail messages can be either delivered immediately or held for later
delivery. Held messages are referred to as
"queued." They are placed into
either a single holding directory (usually called
mqueue) or several directories from which they
are later delivered. There are many reasons a mail message might be
If a mail message is temporarily undeliverable, it is queued and
delivery is attempted later. If the message is addressed to multiple
recipients, it is queued only for those recipients to whom delivery
is not immediately possible.
If the SuperSafe option (SuperSafe) is set to true (the default setting required
by RFC2824), all mail messages are queued for safety while delivery
is attempted. The message is removed from the queue only if delivery
succeeds. If delivery fails, the message is left in the queue, and
another attempt is made to deliver it later. This causes the mail to
be saved in the unhappy event of a system crash during processing.
If sendmail is run with the
DeliveryMode option (DeliveryMode)
set to queue-only or to defer,
all mail is queued, and no immediate delivery attempt is made. A
separate queue run is required to attempt delivery.
If the load (average number of blocked processes) becomes higher than
the value given to the QueueLA option (QueueLA), sendmail will queue a
message rather than attempt to deliver it. A separate queue run is
required later to process the queue.