I've used this command to send spam messages to my test server during the develop of this options, but it will also be useful if you need to resend a quarantined message to the recipient
/var/qmail/bin/qmail-inject email@example.com < name_of_the_file
The return-path of the resent message will be the user who runs this command, unless you give as first argument:
-firstname.lastname@example.orgThe order is mandatory. See man qmail-inject for more details.
As qmail-inject is not in your path it is a good idea to make an alias of it.