sendmail cannot immediately deliver a mail
message, it places that message in a queue to await another try.
Prior to V8.12, the QUEUE definition caused queue-handling code to be
included in sendmail. As of V8.12, the QUEUE
compile-time macro has been removed, and queue-handling code is
always included in sendmail.
If queueing is not enabled and you need to queue,
sendmail prints the following message and either
bounces or discards the message:
A word to the wise: always define QUEUE. Even if
you have only a pure UUCP machine, mail can fail (for a reason such
as a full disk). Without queueing, such mail will bounce when instead
it should be queued for a later try.
The default is to always define QUEUE if NETINET or NETISO are
defined; otherwise, QUEUE is undefined. There is no debugging flag to
show whether QUEUE is defined, but the -bp switch
(Section 11.6) can be used to determine whether it is