From the Qmail FAQ (in /var/qmail/doc/FAQ)
7.2. How do I manually run the queue? I'd like qmail to try delivering all the remote messages right now.
Answer: Give the qmail-send process an ALRM.
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At the command prompt logged in as root, type:
killall -ALRM qmail-send
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You may want to run qmail-tcpok first, to guarantee that qmail-remote will try all addresses. Normally, if an address fails repeatedly, qmail-remote leaves it alone for an hour.
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If you want to know the number of messages in the queue at the command prompt logged in as root, type:
/var/qmail/bin/qmail-qstat