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 queued:
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.
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)
queue-only or to
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.