One last note on this thread. While rereading the FAQ, I came across this
which indicates qmail has brakes to keep from generating denial of service
attacks.
http://cr.yp.to/qmail/faq/efficiency.html
Does qmail back off from dead hosts?
Answer: Yes. qmail has three backoff features:
Each message is automatically retried on a quadratic schedule, with longer
and longer intervals between delivery attempts.
If a remote host does not respond to two connection attempts (separated by
at least two minutes with no intervening successful connections), qmail
automatically leaves the host alone for an hour. At the end of the hour it
``slow-starts,'' allowing one connection through to see whether the host is
up.
Some mailers opportunistically bombard a host with deferred messages as soon
as the host comes back online. qmail does not do this. Each message waits
until the appropriate retry time.
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