How, for the daemon, -odf is exactly the same as -odb?
With -odb, the accepting process doesn't wait for the delivery processes to
finish. With -odf, it does wait. And there is also a difference about
closing the standard error.
$ zgrep -A28 ^-odb /usr/share/doc/exim4/spec.txt.gz | sed '/If one/,+3d'
-odb
This option applies to all modes in which Exim accepts incoming messages,
including the listening daemon. It requests "background" delivery of such
messages, which means that the accepting process automatically starts a
delivery process for each message received, but does not wait for the
delivery processes to finish.
When all the messages have been received, the reception process exits,
leaving the delivery processes to finish in their own time. The standard
output and error streams are closed at the start of each delivery process.
This is the default action if none of the -od options are present.
-odf
This option requests "foreground" (synchronous) delivery when Exim has
accepted a locally-generated message. (For the daemon it is exactly the
same as -odb.) A delivery process is automatically started to deliver the
message, and Exim waits for it to complete before proceeding.
The original Exim reception process does not finish until the delivery
process for the final message has ended. The standard error stream is left
open during deliveries.
