If 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:

dropenvelope: queueup

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 supported.