O DeliveryMode=mode             configuration file (V8.7 and later) 
-ODeliveryMode=mode             command line (V8.7 and later) 
define(`confDELIVERY_MODE',mode)    mc configuration (V8.7 and later) 
Odmode                          configuration file (deprecated) 
-odmode                         command line (deprecated) 

Unknown delivery mode char

need QUEUE to set -odqueue or -oddefer

DeliveryMode=background

Background mode—intended primarily^[20] for use in the configuration file—allows sendmail to run asynchronously. This means that once sendmail has gathered the entire message and verified that the recipient is deliverable, it will fork(3) a copy of itself and exit. The copy, running in the background (asynchronously), will then handle the delivery. From the user's point of view, this mode allows the mail interface program to act as though it sent the message nearly instantaneously.

^[20] A sending program (MUA) might need to use background mode on the command line if the message is urgent and if the default in sendmail's configuration file is to queue all messages (with q mode).

DeliveryMode=deferred

Deferred mode—for use in either the command line or the configuration file—is much like queue-only mode except that all database lookups, including DNS, are deferred until the actual queue run. Deferred mode (V8.7 and later) is preferred for dial-on-demand sites (typically, modem-based SL/IP or PPP connections). Just as in queue-only mode, all mail is queued for later delivery, but with deferred mode, code inside sendmail that would ordinarily interact with DNS is suppressed. This prevents the modem from being dialed and connections from being established every time mail is queued.

See also the -D database-map switch (-D), and the DialDelay option (DialDelay).

DeliveryMode=interactive

Interactive mode—intended for use from the command line—causes sendmail to run synchronously. This mode is useful primarily for debugging mail problems. Instead of going into the background with fork(3), it runs in the foreground (synchronously). In this mode, error messages are printed back to the controlling terminal rather than being mailed to the user as bounced mail. The -v command-line switch (-v) automatically sets the mode to interactive.

DeliveryMode=queueonly

Queue-only mode—for use in either the command line or the configuration file—causes sendmail to synchronously queue mail. Queue-only mode is useful at sites that have huge amounts of UUCP mail or Usenet news batch feeds, or when delivering to low-priority addresses such as mailing lists. Queuing has the beneficial effect of serializing delivery through queue runs, and it reduces the load on a machine that many parallel backgrounded sendmail processes can cause. Queue-only mode is typically supplied as a command-line option to sendmail by the uuxqt(8) program. When queue-only mode is selected, all mail is queued for delivery, and none is actually delivered. A separate run of sendmail with its -q command-line switch (Section 11.8.1) is needed to actually process the queue. Note that addresses can still be looked up with DNS as a part of the queueing process. Consequently, queue-only mode is probably not suitable for dial-on-demand sites.