Most pop servers don't perform deletions until QUIT. If for some reason
the process is killed while fetching a large quantity of messages, the
quit method isn't invoked, while the commit method has been. We may thus
end up with duplicated messages the next time we try to fetch. It therefore
helps to fetch the messages in small subsets and call the quit method between
those subsets.
Julien Legros
318f254761